Deny in former London prison losing weight (6)
I believe the answer is:
negate
'deny' is the definition.
(I've seen this before)
'former london prison losing weight' is the wordplay.
'former london prison' becomes 'newgate' (historical London prison).
'losing' is a deletion indicator.
'weight' becomes 'w' (short for weight).
'newgate' with 'w' taken out is 'NEGATE'.
'in' is the link.
(Other definitions for negate that I've seen before include "Contradict" , "Cause to have no effect" , "disallow!" , "Countermand" , "Disprove or nullify" .)