The Clues – Group 1
Round, crooked grin (4)
— RING = GRIN* ("crooked")
Half-baked trifles accomodating upper class's return (5)
— FRUIT = U (upper class) in (TRIF(les))* ("half-baked", or 4/7-"baked", more like)
Contact the old university heads before church (5)
— TOUCH = T(he) O(ld) U(niversity) + CH (Church)
Mean queen following Penny (6)
— CENTER = CENT (Penny) + ER (Elizabeth Regina)
Floating motion around two different motorways (8)
— SWIMMING = M + MI (two motorways) in SWI.NG
The Clues – Group 2
Youthful graduate at hand (4)
— BABY = B.A. (graduate) + BY (at hand)
Wandering nomad meets unknown from beginning of week (6)
— MONDAY = NOMAD* ("wandering") + Y (typical unknown in algebra)
Crowds in somber assembly back peacekeepers instead of nothing (7)
— NUMBERS = S(O > UN)MBER* ("assembly"; the UN are the peacekeepers)
Lower poles around distant, enclosing gas (8)
— SOUTHERN = H (hydrogen, "gas") in OUT.ER ("distant" ), all in S.N ("poles")
Sour, ripe bananas from manager (8)
— SUPERIOR = (SOUR RIPE)* ("bananas")
The property all words share:
They are all one item in a set of five:
Five fingers on a hand: Thumb, index, middle, ring and little finger
Five groups of food: fruit, vegetables, grains, protein and dairy
The five senses: sight, hearing, smell, touch and taste.
Five basketball positions: center, power forward, small forward, point guard and shooting guard.
Modern pentathlon: fencing, swimming, show jumping, pistol shooting and cross-country running
Five Spice Girls: Scary Spice, Sporty Spice, Baby Spice, Ginger Spice and Posh Spice
Five workdays: Monday, Tuesday, Wednesday, Thursday, Friday
Five Books of Moses: Genesis, Exodus, Leviticus, Numbers and Deuteronomy
Five Oceans: Atlantic Ocean, Pacific Ocean, Indian Ocean, Arctic Ocean and Southern Ocean.
Five Great Lakes: Lake Superior, Lake Michigan, Lake Huron, Lake Erie and Lake Ontario
What differs between the two groups:
The items in Group 2 are proper names and the items in Group 1 are just regular nouns.
High fives to:
hexomino, who solved 2.4.