Being slack, lose out where love's concerned (5)
I believe the answer is:
loose
'being slack' is the definition.
(synonyms)
'lose out where love's concerned' is the wordplay.
I cannot quite understand how this works, but
'love's' could be 'os' (love means zero in tennis) and 'os' is found within the answer.
The remaining letters 'loe' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for loose that I've seen before include "Baggy" , "Unrestrained" , "Not secured" , "Promiscuous" , "Liberal" .)