Promise to stock some prawns inside (5)
I believe the answer is:
spell
'promise' is the definition.
(thesaurus)
'stock some prawns inside' is the wordplay.
'stock' becomes 'sell' ().
'some' says to take the initial letters.
'inside' means one lot of letters goes inside another.
The initial letter of 'prawns' is 'p'.
'sell' placed around 'p' is 'SPELL'.
'to' is the link.
(Other definitions for spell that I've seen before include "something to cast" , "Period; piece of magic" , "Stretch" , "Put letters in right order" , "Identify individual characters" .)