Fair Jordan, initially US tennis's number one (4)
I believe the answer is:
just
'fair' is the definition.
(Synonym)
'jordan initially us tennis's number one' is the wordplay.
'initially' says to take the initial letters.
'tennis's number one' becomes 't' (1st letter of 'tennis').
The initial letter of 'jordan' is 'j'.
'j'+'us'+'t'='JUST'
(Other definitions for just that I've seen before include "By a whisker" , "Fair; a moment ago" , "Barely - deserved" , "Quite" , "Hardly" .)