Obey Lorna, perhaps, by revising study … (2,4,4)

I believe the answer is:

do ones duty

'obey' is the definition.
(eg to follow military orders)

'lorna perhaps by revising study' is the wordplay.
'lorna perhaps' becomes 'Doone' (Lorna Doone is a novel by RD Blackmoore).
'by' means one lot of letters go next to another.
'revising' indicates an anagram.
'study' with letters rearranged gives 'sduty'.
'doone'+'sduty'='DO ONES DUTY'

I've seen this clue in The Guardian.

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