Regular tuna cooked with rocket and leeks for starters (7)
I believe the answer is:
natural
'regular' is the definition.
(thesaurus)
'tuna cooked with rocket and leeks for starters' is the wordplay.
'cooked with' indicates an anagram.
'for starters' indicates taking the first letters.
The first letters of 'rocket and leeks' is 'ral'.
'tuna' anagrammed with 'ral' makes 'NATURAL'.
(Other definitions for natural that I've seen before include "Unaffected - innate" , "Normal - innate - instinctive" , "Organic" , "Unaffected - musical note" , "Not man-made; off-white" .)