It’s awfully hard to hide a navy stain (7)
I believe the answer is:
tarnish
'stain' is the definition.
(I know that tarnish is a type of stain)
'it's awfully hard to hide a navy' is the wordplay.
'awfully' is an anagram indicator.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to hide' indicates putting letters inside.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'its' with letters rearranged gives '[its]'.
'[its]'+'h'='tish'
'a'+'rn'='arn'
'tish' going around 'arn' is 'TARNISH'.
(Other definitions for tarnish that I've seen before include "Stain or discolour, say metal" , "Mountain lake" , "Stain, metal or reputation" , "Film" , "Lose shine" .)