Avoid retirement so as to fill a gap (4,2)
I believe the answer is:
stop up
'fill a gap' is the definition.
'stop up' can be an answer for 'fill' (thesaurus). I'm not sure about the 'a gap' bit.
'avoid retirement so as to' is the wordplay.
I cannot quite see how this works, but
'to' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stop up that I've seen before include "Don't retire here" , "close" , "Don't go to bed early" , "Block (eg an earth)" , "Cork" .)