Measure the hair in need of cutting? (7)
I believe the answer is:
furlong
'measure' is the definition.
(furlong is a kind of measure)
'the hair in need of cutting?' is the wordplay.
'the hair' becomes 'fur' (fur is a kind of hair).
'in need of' is a charade indicator (letters next to each other).
'cutting?' becomes 'long' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fur'+'long'='FURLONG'
Can you help me to learn more?
(Other definitions for furlong that I've seen before include "Imperial unit of length, an eighth of a mile" , "220 yards" , "Measure of distance" , "1/8th of a mile" , "One eighth of a mile" .)