Right one invalid? Doctor visiting sort of can (4-4)

I believe the answer is:

ring-pull

'sort of can' is the definition.

'right one invalid? doctor visiting' is the wordplay.
'right' becomes 'r' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'invalid?' becomes 'null' ('null' can be similar in meaning to 'invalid').
'doctor' becomes 'gp' (general practitioner**).
'visiting' is an insertion indicator.
'null' placed around 'gp' is 'ngpull'.
'r'+'i'+'ngpull'='RING-PULL'

I've seen this clue in The Times.

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