They can be made to measure under the arms (12)

'arms' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps there's an association between them I don't understand?

'they can be made to measure under the' is the wordplay.
I cannot quite understand how this works, but
'measure' could be 'therm' (therm is a kind of measure) and 'therm' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?