Bug dug centrally in timber if really required (2,1,4)
I believe the answer is:
at a push
'if really required' is the definition.
'bug dug centrally in timber' is the wordplay.
'bug' becomes 'tap' (I've seen this before).
'centrally' says to take the centre.
'in' indicates putting letters inside.
'timber' becomes 'ash' (I've seen this before).
The central letter of 'dug' is 'u'.
'tap'+'u'='tapu'
'tapu' put into 'ash' is 'AT A PUSH'.
(Other definitions for at a push that I've seen before include "If a big effort were made - say a shove" , "With difficulty" , "If really necessary" .)