Duplicity as king and queen held by force at sea (7)
I believe the answer is:
knavery
'duplicity' is the definition.
'king and queen held by force at sea' is the wordplay.
'king' becomes 'k'.
'and' says to put letters next to each other.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'held by' means one lot of letters goes inside another.
'force at sea' becomes 'navy'.
'er' inserted within 'navy' is 'navery'.
'k'+'navery'='KNAVERY'
'as' acts as a link.
(Other definitions for knavery that I've seen before include "Deceitful behaviour" , "Cheating behaviour" , "Dishonest behaviour" , "Wickedness" , "fraud" .)