Casual role for father (4,4)

I believe the answer is:

part time

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'casual' is the definition.
(thesaurus)

'role for father' is the wordplay.
'role' becomes 'part' (eg a part or role in a film).
'for father' becomes 'time' (I can't explain this - if you can you should believe this answer much more).
'part'+'time'='PART-TIME'

Can you help me to learn more?

(Other definitions for part time that I've seen before include "temp?" , "One may find some workers like this" , "Not fully committed" , "Not working a full week" .)

I've seen this clue in the Evening Standard.

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