Screw top of jar I dropped into real mess (6)
I believe the answer is:
jailer
'screw' is the definition.
(synonyms)
'top of jar i dropped into real mess' is the wordplay.
'top of' says to take the initial letters.
'dropped into' is an insertion indicator.
'mess' indicates an anagram.
The first letter of 'jar' is 'j'.
'real' is an anagram of 'aler'.
'i' put within 'aler' is 'ailer'.
'j'+'ailer'='JAILER'
(Other definitions for jailer that I've seen before include "one professionally deployed with a pen?" , "Prison guard" , "Old-fashioned screw" , "Custodian of a prisoner" , "Prison warder" .)