One's attractive sweetheart abysmal to calmer gent (13)
I believe the answer is:
electromagnet
'one's attractive' is the definition.
(an electromagnet attracts things)
'sweetheart abysmal to calmer gent' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'abysmal' is an anagram indicator (an abysmal spelling).
'to'+'calmer'+'gent'='tocalmergent'
'tocalmergent' with letters rearranged gives 'lectromagnet'.
'e'+'lectromagnet'='ELECTROMAGNET'
(Other definitions for electromagnet that I've seen before include "attractive item" , "Device using current to attract iron" , "Device attracting items when a current is passed through it" , "Powered attractor" .)