Gained hair untidily that's on the side of the head (7,3)
I believe the answer is:
hearing aid
'that's on the side of the head' is the definition.
(a hearing aid is worn on the side of the head)
'gained hair untidily' is the wordplay.
'untidily' indicates an anagram (I've seen this in other clues).
'gained'+'hair'='gainedhair'
'gainedhair' anagrammed gives 'HEARING-AID'.
(Other definitions for hearing aid that I've seen before include "Help for the partially deaf" , "asset in court?" , "Earpiece" , "Ear trumpet" , "Amplifier for ear" .)