Relationship between two gunners starts to turn in operations (5)
I believe the answer is:
ratio
'relationship' is the definition.
(relationship between two numbers)
'two gunners starts to turn in operations' is the wordplay.
'two gunners' becomes 'ra' (Royal Artillery. I am not sure about the 'two' bit.).
'starts to' indicates taking the first letters.
The initial letters of 'turn in operations' is 'tio'.
'ra'+'tio'='RATIO'
'between' acts as a link.
Can you help me to learn more?
(Other definitions for ratio that I've seen before include "Proportion of one thing to another" , "figures in relationship" , "Relationship found in a riot" , "2:1, for example" , "Mathematical relationship" .)