Wandering mariner helping to protect Queen (10)
I believe the answer is:
aberration
'wandering' is the definition.
(I've seen this before)
'mariner helping to protect queen' is the wordplay.
'mariner' becomes 'AB' (short for able seaman).
'helping' becomes 'ration' (a ration is a serving or helping).
'to protect' is an insertion indicator (inserted letters are held or protected).
'queen' becomes 'ER' (short for Elizabeth Regina ie Queen Elizabeth).
'ab'+'ration'='abration'
'abration' enclosing 'er' is 'ABERRATION'.
(Other definitions for aberration that I've seen before include "Deviation from normality" , "mistake" , "irregularity" , "Slip" , "instability" .)