Expert training teams or retiring (7)
I believe the answer is:
maestro
'expert' is the definition.
(I've seen this before)
'training teams or retiring' is the wordplay.
'training' is an anagram indicator (to train can mean to grow something into a shape).
'retiring' shows that the letters should be reversed in order.
'or' written backwards gives 'ro'.
'teams' anagrammed gives 'maest'.
'maest'+'ro'='MAESTRO'
(Other definitions for maestro that I've seen before include "Distinguished artist" , "Are most in favour of a foreign master?" , "Eminent conductor or composer" , "Some art (anag.)" , "Top conductor" .)