Forgive a cheat in navy after pressure (6)
I believe the answer is:
pardon
'forgive' is the definition.
(I know that pardon is a kind of forgive)
'a cheat in navy after pressure' is the wordplay.
'cheat' becomes 'do'.
'in' indicates putting letters inside.
'navy' becomes 'RN' (Royal Navy).
'after' means one lot of letters go next to another.
'pressure' becomes 'p' (scientific abbreviation).
'do' going into 'rn' is 'rdon'.
'a'+'rdon'='ardon'
'ardon' put after 'p' is 'PARDON'.
(Other definitions for pardon that I've seen before include "Forgive; come again?" , "Grant forgiveness" , "indulgence" , "Let off" , "Forgiveness for an offence" .)