Fellow clan member finally seen in mask, surprisingly (7)
I believe the answer is:
kinsman
'fellow clan member' is the definition.
(a member of one's clan or kin)
'finally seen in mask surprisingly' is the wordplay.
'finally' says to take the final letters.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
The final letter of 'seen' is 'n'.
'n'+'in'+'mask'='ninmask'
'ninmask' anagrammed gives 'KINSMAN'.
(Other definitions for kinsman that I've seen before include "A blood relation" , "(Male) relative" , "Blood relative" , "Family member" , "Male relation" .)