The arm kept up one's sleeve? (6,6)
I believe the answer is:
secret weapon
'up one's sleeve?' is the definition.
I know nothing about this answer so I can't judge whether this works.
'the arm' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'arm' could be 'weapon' (synonyms) and 'weapon' is present in the answer.
This explanation may well be incorrect...
'kept' acts as a link.
Can you help me to learn more?
(Another definition for secret weapon that I've seen is " Pea, sweet corn (anag)".)