Dicky tucks into part of fish substitute (4-2)

I believe the answer is:

fill-in

'substitute' is the definition.
(I know that substitute can be written as fill-in)

'dicky tucks into part of fish' is the wordplay.
'dicky' becomes 'ill' (I've seen this before).
'tucks into' indicates putting letters inside.
'part of fish' becomes 'fin' (fish have fins).
'ill' put inside 'fin' is 'FILL-IN'.

I've seen this clue in The Times.

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