Took up suitable part of the fish for lunch (6)
I believe the answer is:
tiffin
'lunch' is the definition.
(synonyms)
'took up suitable part of the fish' is the wordplay.
'took up' says the letters should be written in reverse (I've seen 'taken up' mean this).
'suitable' becomes 'fit' (similar in meaning).
'part of the fish' becomes 'fin' ('fin' is part of 'fish').
'fit' reversed gives 'tif'.
'tif'+'fin'='TIFFIN'
'for' acts as a link.
(Other definitions for tiffin that I've seen before include "Light Indian repast" , "Light midday meal in India" , "Midday meal for members of the British Raj in India" , "Light (Indian) meal" , "Snack, light meal (dated)" .)