Measure last of Scotch after some maths backfired (5)
I believe the answer is:
girth
'measure' is the definition.
(I've seen this before)
'last of scotch after some maths backfired' is the wordplay.
'last of' indicates one should take the final letters.
'after' means one lot of letters go next to another.
'some maths' becomes 'trig' (trig is a kind of maths).
'backfired' says the letters should be written backwards.
The last letter of 'scotch' is 'h'.
'h'+'trig'='htrig'
'htrig' back-to-front is 'GIRTH'.
(Other definitions for girth that I've seen before include "Circumferential measure" , "Circumpherence" , "Body's circumference around the middle" , "The distance around a thing or tummy" , "Right (anag.)" .)