Unusual, small gym class US agents left (7)
I believe the answer is:
special
'unusual' is the definition.
(I've seen this before)
'small gym class us agents left' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'gym class' becomes 'pe' (abbreviation for physical education).
'us agents' becomes 'CIA' (CIA, Central Intelligence Agency).
'left' becomes 'l' (common abbreviation).
's'+'pe'+'cia'+'l'='SPECIAL'
(Other definitions for special that I've seen before include "Rare, valuable" , "Dish not on regular menu" , "Dish not on main menu" , "Distinctive, exceptional" , "Different from usual" .)