Convincing the Yorkshire pianist to shed weight (7)
I believe the answer is:
telling
'convincing' is the definition.
(thesaurus)
'the yorkshire pianist to shed weight' is the wordplay.
'the yorkshire' becomes 't' (Northern English pronunciation of 'the').
'pianist' becomes 'ellington' (jazz pianist Duke Ellington).
'to shed' is a deletion indicator (some letters are shed or discarded).
'weight' becomes 'ton' (unit of weight).
'ellington' with 'ton' taken away is 'elling'.
't'+'elling'='TELLING'
(Other definitions for telling that I've seen before include "relation" , "Providing information" , "Striking, having noticeable effect" , "Powerful - effective" , "Effective - narration" .)