Idiot handling small, unexpected development (5)
I believe the answer is:
twist
'unexpected development' is the definition.
(I've seen this before)
'idiot handling small' is the wordplay.
'idiot' becomes 'twit' (I've seen this before).
'handling' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'twit' placed around 's' is 'TWIST'.
(Other definitions for twist that I've seen before include "Convolute" , "Tobacco - fraud - unexpected development - dance" , "Screw; distort" , "Oliver --, Dickens character" , "One who wanted more" .)