Sentimentality is mostly beneficial (3)
I believe the answer is:
goo
'sentimentality' is the definition.
(informal term for sentimentality)
'mostly beneficial' is the wordplay.
'mostly' means to remove the last letter.
'beneficial' becomes 'good' ('good' can be a synonym of 'beneficial').
'good' with its final letter taken away is 'GOO'.
'is' acts as a link.
(Other definitions for goo that I've seen before include "Sticky and slimy stuff" , "Sticky substance (colloq.)" , "Sentimentality (sl.)" , "Sticky or slimy substance" , "Sticky gunge" .)