Singing John, the Spanish heavyweight (5)
I believe the answer is:
elton
'singing john' is the definition.
(I've seen this in another clue)
'the spanish heavyweight' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'heavyweight' becomes 'ton' (a ton weighs a lot).
'el'+'ton'='ELTON'
(Other definitions for elton that I've seen before include "Let on he's John, the singer" , "One John" , "First name of singer/songwriter born Reginald Dwight in 1947" , "Sir - - John, singer songwriter" , "Sir ___ John, singer-songwriter" .)