Knock through loose chain if necessary (2,1,5)
I believe the answer is:
at a pinch
'if necessary' is the definition.
(I've seen this before)
'knock through loose chain' is the wordplay.
'knock' becomes 'tap' (synonyms).
'through' is an insertion indicator (I've seen this in other clues).
'loose' indicates an anagram (letters are loosened from their original positions).
'chain' with letters rearranged gives 'ainch'.
'tap' placed inside 'ainch' is 'AT A PINCH'.
(Other definitions for at a pinch that I've seen before include "In case of necessity" , "If absolutely necessary" , "if need be" , "In time of difficulty or necessity - nip it" , "in a crisis?" .)