Play into each other's hands in using an old clue (7)
I believe the answer is:
collude
'play into each other's hands' is the definition.
I can't judge whether this definition defines the answer.
'using an old clue' is the wordplay.
'using' is an anagram indicator (the letters used in a different way).
'old'+'clue'='oldclue'
'oldclue' with letters rearranged gives 'COLLUDE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for collude that I've seen before include "Conspire fraudulently" , "Conspire to defraud" , "Be in cahoots" , "Agree in secret" , "Come to a secret and devious understanding" .)