Work's been mounted, look … as sport (4)
I believe the answer is:
polo
'sport' is the definition.
(polo is a kind of sport)
'work's been mounted look' is the wordplay.
'work's been' becomes 'op' (I can't justify this - if you can you should believe this answer much more).
'mounted' says the letters should be written in reverse.
'look' becomes 'lo'.
'op' reversed gives 'po'.
'po'+'lo'='POLO'
'as' is the link.
Can you help me to learn more?
(Other definitions for polo that I've seen before include "A mint on horseback?" , "Game played on horseback wielding mallets" , "Equestrian game" , "Sweet" , "Horseback sport" .)