Twerp in film, deep in thought (10)

I believe the answer is:

cogitating

'deep in thought' is the definition.
(cogitating means thinking hard about something)

'twerp in film' is the wordplay.
'twerp' becomes 'git' (both can mean an annoying person).
'in' is an insertion indicator.
'film' becomes 'coating' (film can mean a coating or layer).
'git' going into 'coating' is 'COGITATING'.

I've seen this clue in The Guardian.

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