Fuss when commercial is on loop (3)
I believe the answer is:
ado
'fuss' is the definition.
(synonyms)
'commercial is on loop' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'is on' means one lot of letters go next to another (in a down clue, letters appear on others).
'loop' becomes 'O' (O looks like a loop).
'ad'+'o'='ADO'
'when' acts as a link.
(Other definitions for ado that I've seen before include "stew is provided" , "'Much ... About Nothing, Shakespeare play (3)'" , "Pother" , "flapping" , "Rapid bustling commotion" .)