So-so vestment hung up on first hook (4)
I believe the answer is:
blah
'so-so' is the definition.
(I've seen this in another clue)
'vestment hung up on first hook' is the wordplay.
'vestment' becomes 'alb' (alb is a kind of vestment).
'hung up' shows that the letters should be reversed in order.
'on' says to put letters next to each other.
'first hook' becomes 'h' (1st letter of 'hook').
'alb' reversed gives 'bla'.
'bla'+'h'='BLAH'
(Other definitions for blah that I've seen before include "Dull" , "Rhubarb" , "Stupid 20" , "Insipid" , "Pretentious nonsense" .)