Yours truly will speak for Sheppey say (4)
I believe the answer is:
isle
'sheppey say' is the definition.
I can't judge whether this defines the answer.
'yours truly will speak' is the wordplay.
'yours truly' becomes 'I' (the person that set this crossword).
'will' becomes ''ll' (contracted form, as in 'he'll' - 'he will').
'speak' indicates a 'sounds like' (homophone) clue.
'i'+'ll'='ill'
'ill' is a homophone of 'ISLE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for isle that I've seen before include "Dogs, say" , "One surrounded by water" , "See 11 Across" , "like 'Hoy!'" , "Capri or Man, perhaps?" .)