Mum always very thin (5)
I believe the answer is:
sheer
'very thin' is the definition.
(I've seen this before)
'mum always' is the wordplay.
'mum' becomes 'sh' (both can mean 'keep quiet').
'always' becomes 'eer' (poetic way of saying ever).
'sh'+'eer'='SHEER'
(Other definitions for sheer that I've seen before include "Arrant" , "Clear" , "needing to change a course" , "Unmitigated; perpendicular" , "Deviate (from) - vertical" .)