They include Mike and Oscar in their letters (4)
I believe the answer is:
nato
'letters' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'they include mike and oscar in' is the wordplay.
I cannot really see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'oscar' could be 'o' (phonetic alphabet: alpha, bravo, charlie etc.) and 'o' is found within the answer.
The remaining letters 'at' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'their' is the link.
Can you help me to learn more?
(Other definitions for nato that I've seen before include "Western mil. alliance" , "International organisation for collective security" , "Military/political alliance" , "Atlantic alliance (abbr.)" , "Transatlantic alliance" .)