Concealed lid on top of tin (6)
I believe the answer is:
covert
'concealed' is the definition.
(similar in meaning)
'lid on top of tin' is the wordplay.
'lid' becomes 'cover' (I've seen this in another clue).
'on top of' indicates taking the first letters.
The first letter of 'tin' is 't'.
'cover'+'t'='COVERT'
(Other definitions for covert that I've seen before include "Unacknowledged; thicket" , "Not open; thicket" , "Hidden - thicket" , "Thicket; not displayed" , "A fox's retreat, secret" .)