Surprise leading group (5)
I believe the answer is:
upset
'surprise' is the definition.
(I've seen this before)
'leading group' is the wordplay.
'leading' becomes 'up' (similar in meaning).
'group' becomes 'set' (both can mean a collection).
'up'+'set'='UPSET'
(Other definitions for upset that I've seen before include "Disordered - unexpected defeat" , "Capsize" , "Overturn - worry" , "Disturbed or inverted" , "Overturn, tip over" .)