Star culture mostly irregular of form (10)
I believe the answer is:
structural
'of form' is the definition.
(relating to structure or form)
'star culture mostly irregular' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'irregular' indicates anagramming the letters.
'star'+'culture'='starculture'
'starculture' with its final letter removed is 'starcultur'.
'starcultur' is an anagram of 'STRUCTURAL'.
(Other definitions for structural that I've seen before include "Concerned with building" , "Relating to frame of a building" , "Relating to framework" .)