True ally worried about love (5)
I believe the answer is:
loyal
'true' is the definition.
(I've seen this in another clue)
'ally worried about love' is the wordplay.
'worried' is an anagram indicator.
'about' indicates putting letters inside.
'love' becomes 'o' (love means zero in tennis).
'ally' anagrammed gives 'lyal'.
'lyal' placed around 'o' is 'LOYAL'.
(Other definitions for loyal that I've seen before include "Faithful (in allegiance)" , "True-hearted" , "True to one's allegiance" , "Showing firm allegiance" , "Showing enduring firm support" .)