Declare invalid's wife released from old London gaol (6)
I believe the answer is:
negate
'declare invalid's' is the definition.
'wife released from old london gaol' is the wordplay.
'wife' becomes 'w'.
'released from' indicates named letters should be taken away.
'old london gaol' becomes 'newgate'.
'newgate' with 'w' removed is 'NEGATE'.
(Other definitions for negate that I've seen before include "Render ineffective" , "Age ten (anag)" , "Cancel; invalidate" , "Discredit" , "Opera by Puccini" .)