Short and interesting, at heart (5)
I believe the answer is:
pithy
'short' is the definition.
(thesaurus)
'interesting at heart' is the wordplay.
I cannot quite understand how this works, but
'heart' could be 'pith' (synonyms) and 'pith' is present in the answer.
A single letter 'y' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'and' acts as a link.
Can you help me to learn more?
(Other definitions for pithy that I've seen before include "Terse and vigorously expressive" , "Short and sweet" , "Cogent" , "Terse, powerful (speech)" , "Brief, to the point" .)