Behind short film supporting revised rate for hearing aid (3,7)
I believe the answer is:
ear trumpet
'hearing aid' is the definition.
(synonyms)
'behind short film supporting revised rate' is the wordplay.
'behind' becomes 'rump' (informal term for the buttocks).
'short film' becomes 'et' (short title of E.T. the Extra-Terrestrial).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'revised' indicates an anagram.
'rate' anagrammed gives 'eart'.
'rump'+'et'='rumpet'
'rumpet' put after 'eart' is 'EAR-TRUMPET'.
'for' acts as a link.
(Other definitions for ear trumpet that I've seen before include "One gathering sound" , "eight [HEARING AID]" , "sound engineering device" , "Primitive hearing aid" , "Early hearing aid" .)