Leading group capsized (5)
I believe the answer is:
upset
'capsized' 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'
(Other definitions for upset that I've seen before include "Disturb the balance or stability of" , "bug" , "Indisposition" , "Surprising result" , "Caused grief" .)