Listen to a number cheer (7)
I believe the answer is:
hearten
'cheer' is the definition.
('hearten' can be a synonym of 'cheer')
'listen to a number' is the wordplay.
'listen to' becomes 'hear' (I've seen this before).
'a number' becomes 'ten' (ten is a kind of number).
'hear'+'ten'='HEARTEN'
(Other definitions for hearten that I've seen before include "Make cheerful" , "Inspirit" , "Encourage or cheer in cardiac manner" , "Buoy" , "Encourage in Teheran" .)