One gesture of a number on the quarter deck (3)
I believe the answer is:
nod
'one gesture' is the definition.
(nod is a kind of gesture)
'of a number on the quarter deck' is the wordplay.
'of' becomes 'o''.
'a number' becomes 'n'.
'on' means one lot of letters go next to another.
'the quarter deck' becomes 'd' (I can't justify this - if you can you should believe this answer much more).
'o' put after 'n' is 'no'.
'no'+'d'='NOD'
Can you help me to learn more?
(Other definitions for nod that I've seen before include "It's said to be as good as a wink" , "overlook something" , "Sign of assent or salutation" , "Gesture of acceptance" , "Drop the head" .)