Sporting American in tree (4)
I believe the answer is:
fair
'sporting' is the definition.
(similar in meaning)
'american in tree' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'in' is an insertion indicator.
'tree' becomes 'fir' (fir is a kind of tree).
'a' put into 'fir' is 'FAIR'.
(Other definitions for fair that I've seen before include "(Of weather) fine and dry" , "'Just, meet (4)'" , "(Of skin) pale" , "legitimate" , "the place for a roundabout" .)