Rhythm provided at heart nice, blissful (8)
I believe the answer is:
beatific
'blissful' is the definition.
(I've seen this in another clue)
'rhythm provided at heart nice' is the wordplay.
'rhythm' becomes 'beat' (synonyms).
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'at heart' says to take the centre.
The middle letters of 'nice' are 'ic'.
'beat'+'if'+'ic'='BEATIFIC'
(Other definitions for beatific that I've seen before include "Feeling or expressing blissful happiness" , "Saintly, blessed" , "Befit CIA to be showing great happiness" , "Angelic, saintlike" , "Showing great happiness, perhaps a smile" .)