No power to take up in tired arms (8)
I believe the answer is:
weaponry
'arms' is the definition.
(synonyms)
'no power to take up in tired' is the wordplay.
'power' becomes 'p' (symbol in physics).
'to take up' shows that the letters should be reversed in order (I've seen 'taken up' mean this).
'in' means one lot of letters goes inside another.
'tired' becomes 'weary' (similar in meaning).
'no'+'p'='nop'
'nop' written backwards gives 'pon'.
'pon' put within 'weary' is 'WEAPONRY'.
(Other definitions for weaponry that I've seen before include "fighting equipment" , "Big guns" , "Arms; artillery" , "Arsenal" , "All one's arms" .)