Better thus to pen extremely leisurely song (7)
I believe the answer is:
calypso
'song' is the definition.
(I know that Calypso is a song)
'better thus to pen extremely leisurely' is the wordplay.
'better' becomes 'cap' (both can mean to beat somebody else's attempt).
'thus' becomes 'so' (synonyms).
'to pen' indicates putting letters inside.
'extremely' suggests removing the centre (only the extremes of the word are used).
'leisurely' with its centre taken out is 'ly'.
'cap'+'so'='capso'
'capso' going around 'ly' is 'CALYPSO'.
(Other definitions for calypso that I've seen before include "Sea nymph of Greek mythology - Caribbean ballad" , "island song" , "West Indian ballad - Homerian sea nymph" , "West Indian ballad rhythm" , "A number of West Indians" .)