Leading group in distress (5)
I believe the answer is:
upset
'distress' is the definition.
(thesaurus)
'leading group' is the wordplay.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'group' becomes 'set' (set is a kind of group).
'up'+'set'='UPSET'
'in' acts as a link.
(Other definitions for upset that I've seen before include "turned over" , "Discountenanced" , "unexpectedly beaten?" , "'Disturbed, agitated (5)'" , "Out of sorts" .)