Dark and almost monstrous at heart (5)
I believe the answer is:
night
'dark' is the definition.
(night is a kind of dark)
'almost monstrous at heart' is the wordplay.
'almost' becomes 'nigh' ('nigh' can be a synonym of 'almost').
'at heart' indicates the central letters.
The central letter of 'monstrous' is 't'.
'nigh'+'t'='NIGHT'
'and' is the link.
(Other definitions for night that I've seen before include "Ignorance" , "Time between days" , "Time of darkness" , "follow-up to 16 [TWILIGHT]" , "Illumination needed now" .)