Go off fish served up for cook (6)
I believe the answer is:
doctor
'for cook' is the definition.
(I've seen this before)
'go off fish served up' is the wordplay.
'go off' becomes 'rot' (as in food going off).
'fish' becomes 'cod' (type of fish).
'served up' says the letters should be written in reverse (in a down clue, going upwards).
'rot'+'cod'='rotcod'
'rotcod' written backwards gives 'DOCTOR'.
(Other definitions for doctor that I've seen before include "Medic or holder of highest university degree" , "Health expert" , "bones" , "Foster, Who, Jekyll or Livingstone" , "GP" .)