Almost explode, seeing blunder (4)
I believe the answer is:
goof
'blunder' is the definition.
('goof' can be a synonym of 'blunder')
'almost explode' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'explode' becomes 'gooff' (going off is a kind of exploding).
'gooff' with its last letter removed is 'GOOF'.
'seeing' acts as a link.
(Other definitions for goof that I've seen before include "foolish error" , "In slang, bungle or blunder" , "Foolish mistake" , "(Make a) silly mistake" , "boob" .)