Say, when you heard about custom (5)
I believe the answer is:
usage
'custom' is the definition.
(synonyms)
'say when you heard about' is the wordplay.
'say' becomes 'eg'.
'when' becomes 'as'.
'heard' indicates a 'sounds like' (homophone) clue.
'about' shows that the letters should be reversed in order.
'you' is a homophone of 'u'.
'eg'+'as'+'u'='egasu'
'egasu' written backwards gives 'USAGE'.
(Other definitions for usage that I've seen before include "Acceptable language" , "Way of employing" , "Habitual custom" , "Treatment; tradition" , "Tradition - rate of consumption" .)