The look of one left for ever? (4)
I believe the answer is:
leer
'the look' is the definition.
(leer is a kind of look)
'one left for ever?' is the wordplay.
'one left' becomes 'l' (common abbreviation).
'for' means one lot of letters go next to another.
'ever?' becomes 'eer' ('e'er' can be a synonym of 'ever').
'l'+'eer'='LEER'
'of' acts as a link.
(Other definitions for leer that I've seen before include "unpleasant look" , "Lecherous look" , "Ogle, smirk" , "Suggestive or sneering grin" , "Look lasciviously at" .)