Beast with diamonds holds hearts back (7)
I believe the answer is:
echidna
'beast' is the definition.
(echidna is a kind of beast)
'with diamonds holds hearts back' is the wordplay.
'with' becomes 'and'.
'diamonds' becomes 'ice' (slang term for diamonds).
'holds' indicates putting letters inside.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'back' is a reversal indicator.
'and'+'ice'='andice'
'andice' placed around 'h' is 'andihce'.
'andihce' written backwards gives 'ECHIDNA'.
(Other definitions for echidna that I've seen before include "Australian creature" , "One preying on the workers" , "Had nice (anag) -- spiny anteater" , "Little animal" , "Creature with spines" .)