Expert commercial for retro gym calls time (5)
I believe the answer is:
adept
'expert' is the definition.
(synonyms)
'commercial for retro gym calls time' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'for' means one lot of letters go next to another.
'retro' is a reversal indicator (retro is Latin for 'backwards').
'gym' becomes 'pe' (short for Physical Education).
'calls time' becomes 't' (I can't justify this - if you can you should believe this answer much more).
'pe' reversed gives 'ep'.
'ad'+'ep'+'t'='ADEPT'
Can you help me to learn more?
(Other definitions for adept that I've seen before include "Deacon" , "Practised" , "Showing skill or aptitude" , "Very skilled person" , "Master" .)