Sentimental stuff is fine, mostly (3)
I believe the answer is:
goo
'sentimental stuff' is the definition.
(goo can mean sentimental or soppy material)
'fine mostly' is the wordplay.
'fine' becomes 'good' (I've seen this before).
'mostly' means to remove the last letter.
'good' with its final letter taken off is 'GOO'.
'is' is the link.
(Other definitions for goo that I've seen before include "Anything sticky" , "Sentimentality (sl.)" , "Sticky or slimy substance" , "Sticky substance (colloq.)" , "syrup" .)