Answer: draw off small measure, if necessary (2,1,5)
I believe the answer is:
at a pinch
'if necessary' is the definition.
(at a pinch means if required)
'answer draw off small measure' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'draw off' becomes 'tap' (I've seen this in another clue).
'small measure' becomes 'inch' (small length measurement).
'a'+'tap'+'inch'='AT A PINCH'
(Other definitions for at a pinch that I've seen before include "if need be" , "If really necessary" , "In time of difficulty or necessity - nip it" , "In case of necessity" , "under pressure" .)