Source of leak found during extensive search (5)
I believe the answer is:
rifle
'search' is the definition.
(thesaurus)
'source of leak found during extensive' is the wordplay.
'source of' indicates taking the first letters.
'found during' means one lot of letters goes inside another.
'extensive' becomes 'rife'.
The initial letter of 'leak' is 'l'.
'l' going within 'rife' is 'RIFLE'.
(Other definitions for rifle that I've seen before include "lock, stock and barrel" , "Lifer (anag.)" , "Strip - firearm" , "loot in sack" , "Carbine" .)