To alienate sweetheart would be peculiar (8)
I believe the answer is:
estrange
'to alienate' is the definition.
(synonyms)
'sweetheart would be peculiar' is the wordplay.
I cannot really see how this works, but
'sweetheart' could be 'e' (the middle letter or heart of 'sweet') and 'e' is present in the answer.
'be' could be 'range' (ranging is a kind of being) and 'range' is found within the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for estrange that I've seen before include "Get cut off" , "Cause to be alienated" , "Ten gears (anag.)" , "Cause to become unfriendly using reagents" , "may break up" .)