Treats a spy as a mercenary? (4)

I believe the answer is:

pays

I'm a little stuck... Click here to teach me more about this clue!

'as a mercenary?' is the definition.
I can't tell whether this defines the answer.

'treats a spy' is the wordplay.
'treats' indicates an anagram.
'a'+'spy'='aspy'
'aspy' anagrammed gives 'PAYS'.

Can you help me to learn more?

(Other definitions for pays that I've seen before include "Gives money (for goods, etc)" , "stumps up" , "Benefits" , "Gives money for work" , "Imburses" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app