Hard bit of maths — set about finding measurement (5)
I believe the answer is:
Here is my best explanation:
'measurement' is the definition.
'hard bit of maths set about' is the wordplay.
'bit of' says to take the initial letters.
'maths' becomes 'trig' (trig is a kind of maths).
'set about' says the letters should be written backwards.
The initial letter of 'hard' is 'h'.
'htrig' back-to-front is 'GIRTH'.
'finding' acts as a link.
(Other definitions for girth that I've seen before include "Right (anag.)", "weighty indication?", "Measure all round", "Bulk", "Perimeter", "target", "Some tack", "Middle", "The distance around a thing or tummy" and "A band passing under horse's belly".)