Love uniform when in senior rank (4)
I believe the answer is:
sour
'rank' is the definition.
(thesaurus)
'love uniform when in senior' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'when in' is an insertion indicator.
'senior' becomes 'sr' ('sr.' can be similar in meaning to 'senior').
'o'+'u'='ou'
'ou' going within 'sr' is 'SOUR'.
(Other definitions for sour that I've seen before include "Curdled" , "Tart, acidic" , "Tasting like 1 Dn" , "Rancid or tart" , "Vinegary" .)