Posh Royal marine overwhelmed by hard work and disruption (7)
I believe the answer is:
turmoil
'disruption' is the definition.
(I know that turmoil is a type of disruption)
'posh royal marine overwhelmed by hard work' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'royal marine' becomes 'RM' (abbreviation).
'overwhelmed by' means one lot of letters goes inside another (inserted letters get absorbed or overwhelmed).
'hard work' becomes 'toil' (I've seen this before).
'u'+'rm'='urm'
'urm' placed inside 'toil' is 'TURMOIL'.
'and' is the link.
(Other definitions for turmoil that I've seen before include "upset" , "Brouhaha" , "Sturm und Drang" , "Hubbub" , "Disquiet" .)