Copper, around a junction, promises to be wary (8)
I believe the answer is:
cautious
'be wary' is the definition.
(similar in meaning)
'copper around a junction promises' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'around' indicates putting letters inside.
'junction' becomes 't'.
'promises' becomes 'IOUs' (promises to pay a debt).
'cu' going around 'a' is 'cau'.
'cau'+'t'+'ious'='CAUTIOUS'
'to' acts as a link.
(Other definitions for cautious that I've seen before include "Wary or prudent" , "Unadventurous" , "Careful, shrewd" , "Careful to avoid danger" , "Careful to avoid dangers" .)