If really necessary, chap will get drunk with a tin (2,1,5)
I believe the answer is:
at a pinch
'if really necessary' is the definition.
(eg something might work 'at a pinch')
'chap will get drunk with a tin' is the wordplay.
'will get drunk with' is an anagram indicator.
'a'+'tin'='atin'
'chap' anagrammed with 'atin' gives 'AT A PINCH'.
(Other definitions for at a pinch that I've seen before include "In time of difficulty or necessity - nip it" , "under pressure" , "in case of emergency" , "if need be" , "In case of necessity" .)