They may hold one in affection (4)
I believe the answer is:
arms
'affection' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'they may hold one' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
The remaining letters 'rms' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for arms that I've seen before include "See 12" , "Limbs of war" , "International competition for weapons" , ". . . . and The Man (G.B. Shaw)" , "bombs and guns etc" .)