Fixing old man caught in whatsit (8)
I believe the answer is:
patching
'fixing' is the definition.
(patching is a kind of fixing)
'old man caught in whatsit' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'caught' becomes 'c' (cricket abbreviation).
'in' is an insertion indicator.
'whatsit' becomes 'thing'.
'c' inserted into 'thing' is 'tching'.
'pa'+'tching'='PATCHING'
(Other definitions for patching that I've seen before include "Mending" , "Nightcap (anag)" .)