Nick, having very old wife, finally says: 'Enter a nunnery!' (4,4)

'enter a nunnery' is the definition.
(a woman takes vows to become a nun)

'nick having very old wife finally says' is the wordplay.
I cannot quite see how this works, but
'very' could be 'v' (abbreviation) and 'v' is present in the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is present in the answer.
'wife' could be 'w' and 'w' is found in the answer.
The remaining letters 'takes' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...