Popular painting etc — a clue: it surprisingly is hard to understand (12)
I believe the answer is:
inarticulate
'hard to understand' is the definition.
(I've seen this before)
'popular painting etc a clue it surprisingly' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'painting etc' becomes 'art' (painting is an art).
'surprisingly' indicates anagramming the letters (a surprising ordering of the letters).
'a'+'clue'+'it'='aclueit'
'aclueit' with letters rearranged gives 'iculate'.
'in'+'art'+'iculate'='INARTICULATE'
'is' acts as a link.
(Other definitions for inarticulate that I've seen before include "I taunt a relic (anag.)" , "incoherent" , "Not able to speak coherently" , "Unintelligible" , "Lacking fluency" .)