Look unhappy when a female breaks Leslie's heart (5)
I believe the answer is:
scowl
'look' is the definition.
(I have seen 'dirty look' mean 'scowl' so perhaps 'look' could also mean 'scowl')
'female breaks leslie's heart' is the wordplay.
'female' becomes 'cow' (I've seen this before).
'breaks' indicates putting letters inside (inserted letters break into the word).
'heart' indicates the central letters.
The central letters of 'leslie' are 'sl'.
'cow' placed inside 'sl' is 'SCOWL'.
'unhappy when a' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for scowl that I've seen before include "Angrily frown" , "Glare" , "Gloomy or threatening expression" , "Grimace" , "Angry frown" .)