Headdresses trade union leader finally proscribes (7)
I believe the answer is:
turbans
'headdresses' is the definition.
(turban is a kind of headdress)
'trade union leader finally proscribes' is the wordplay.
'trade union' becomes 'TU' (abbreviation for trade union).
'finally' suggests the final letters.
'proscribes' becomes 'bans' (banning is a kind of proscribing).
The final letter of 'leader' is 'r'.
'tu'+'r'+'bans'='TURBANS'
(Other definitions for turbans that I've seen before include "Sikh head coverings" , "Eastern head-coverings" .)