Dish of mashed pea all served up (6)
I believe the answer is:
paella
'dish' is the definition.
(type of meal)
'mashed pea all served up' is the wordplay.
'mashed' is an anagram indicator.
'served up' is a reversal indicator (in a down clue, going upwards).
'all' reversed gives 'lla'.
'pea' anagrammed gives 'pae'.
'pae'+'lla'='PAELLA'
'of' is the link.
(Other definitions for paella that I've seen before include "Spanish course" , "Span. rice dish" , "Spanish dish based on saffron rice" , "Saffron-flavoured rice dish" , "Spanish dish of rice, chicken etc cooked in large pan" .)