Martha occasionally accompanying American in pub, if necessary (2,1,4)
I believe the answer is:
at a push
'if necessary' is the definition.
(I've seen this before)
'martha occasionally accompanying american in pub' is the wordplay.
'occasionally' means one should take alternating letters.
'accompanying' is a charade indicator (letters next to each other).
'american' becomes 'us'.
'in' means one lot of letters goes inside another.
'pub' becomes 'ph' (public house).
The alternating letters of 'martha' are 'ata'.
'us' placed into 'ph' is 'push'.
'ata'+'push'='AT A PUSH'
(Other definitions for at a push that I've seen before include "If really necessary" , "With difficulty" , "If a big effort were made - say a shove" .)