Mary adjusted Luke's untidy skullcap (8)
I believe the answer is:
yarmulke
'skullcap' is the definition.
(yarmulke is a kind of skullcap)
'mary adjusted luke's untidy' is the wordplay.
'adjusted' is an anagram indicator.
'untidy' is an anagram indicator.
'mary' is an anagram of 'yarm'.
'luke' anagrammed gives 'ulke'.
'yarm'+'ulke'='YARMULKE'
(Other definitions for yarmulke that I've seen before include "Jewish skullcap; leaky rum (anag.)" , "A skullcap worn by Jewish men" , "Jewish headgear" , "A Jewish man's skullcap" , "Skullcap worn by Jewish man" .)