Listen repeatedly for a sign of approval (4,4)
I believe the answer is:
hear hear
'a sign of approval' is the definition.
'hear hear' can be an answer for 'approval' (I have seen 'express approval' mean 'hear hear' so perhaps 'approval' could also mean 'hear hear'). I am not sure about the 'a sign of' bit.
'listen repeatedly' is the wordplay.
'listen' becomes 'hear' (I've seen this in another clue).
'repeatedly' indicates the doubling of some letters.
'hear' doubled is 'HEAR HEAR'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for hear hear that I've seen before include "Listen, listen - it's an endorsement" , "Listen, listen - it's approval" , "That's right" , "Show of approval" , "Agreed" .)