To an extent resemble Martian sign (6)
I believe the answer is:
emblem
'sign' is the definition.
(emblem is a kind of sign)
'to an extent resemble martian' is the wordplay.
'to an extent' indicates the answer is hidden within the clue (I've seen 'extent of' mean this (I've seen this in other clues)).
'EMBLEM' can be found hidden inside 'resemble martian'.
(Other definitions for emblem that I've seen before include "Visible symbol of an abstract idea" , "Symbolic device or badge" , "Badge, token" , "Symbol, representing a country say" , "Symbol, device that represents something" .)