Vegetable put, essentially, by one of Irving's characters, in salt water (5,3)

'vegetable' is the definition.
(sugar pea is a kind of vegetable)

'put essentially by one of irving's characters in salt water' is the wordplay.
'essentially' indicates the central letters.
'by' is a charade indicator (letters next to each other).
'one of irving's characters' becomes 'Garp' (fictional character created by John Irving).
'in' is an insertion indicator.
'salt water' becomes 'sea' (the sea is made up of salt water).
The middle of 'put' is 'u'.
'u'+'garp'='ugarp'
'ugarp' going into 'sea' is 'SUGAR PEA'.