Eccentric group about to step on rat (5,5)
I believe the answer is:
space cadet
'eccentric' is the definition.
(space cadet can mean an eccentric person)
'group about to step on rat' is the wordplay.
I cannot really understand how this works, but
'group' could be 'a' (A is an example) and 'a' is present in the answer.
'about' could be 'ca' (abbreviation for 'circa') and 'ca' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for space cadet that I've seen before include "one who often gets high" , "one high on drugs" , "trainee astronaut" .)