Is concealing most of sweetheart's thoughts (5)
I believe the answer is:
ideas
'thoughts' is the definition.
(I know this)
'is concealing most of sweetheart's' is the wordplay.
'concealing' is an insertion indicator.
'most of' means to remove the last letter.
'sweetheart' becomes 'dear' (both are terms of address to romantic partner).
'dear' with its last letter taken off is 'dea'.
'is' enclosing 'dea' is 'IDEAS'.
(Other definitions for ideas that I've seen before include "Theories" , "Thoughts, maybe creative" , "Thoughts, creative perhaps" , "Thoughts or opinions" , "Thoughts, perceptions" .)