Partner beginning to lie too much (5)
I believe the answer is:
lover
'partner' is the definition.
(romantic partner)
'beginning to lie too much' is the wordplay.
'beginning to' says to take the initial letters.
'too much' becomes 'over' (over can mean to too great an extent).
The first letter of 'lie' is 'l'.
'l'+'over'='LOVER'
(Other definitions for lover that I've seen before include "finished litre first" , "Boyfriend" , "One enjoys" , "Swain" , "Sweetheart" .)