Ponders brief massage — setter's back and bum! (9)
I believe the answer is:
ruminates
'ponders' is the definition.
('ruminate' can be a synonym of 'ponder')
'brief massage setter's back and bum' is the wordplay.
'brief' means to remove the last letter.
'massage' becomes 'rub' (I've seen this before).
'setter's back' becomes 'mi' (I am not sure about this - if you are sure you should believe this answer much more).
'and' is a charade indicator (letters next to each other).
'bum' becomes 'nates' (anatomical term for the buttocks).
'rub' with its final letter taken off is 'ru'.
'ru'+'mi'+'nates'='RUMINATES'
(Other definitions for ruminates that I've seen before include "Chews the cud and ponders" , "Considers" .)