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.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is found within the answer.
'number' could be 'd' (D is an example) and 'd' is present in the answer.
'quarter' could be 'n' and 'n' is located in the answer.
No letters remain.
This explanation may well be incorrect...
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" .)