Nearly explode finding mistake (4)
I believe the answer is:
goof
'mistake' is the definition.
(I've seen this before)
'nearly explode' is the wordplay.
'nearly' means to remove the last letter.
'explode' becomes 'go off' (going off is a kind of exploding).
'gooff' with its final letter taken away is 'GOOF'.
'finding' is the link.
(Other definitions for goof that I've seen before include "boob" , "(Make a) silly mistake" , "foolish error" , "Blunder, mistake (slang)" , "In slang, bungle or blunder" .)