Perceive a number cheer (7)
I believe the answer is:
hearten
'cheer' is the definition.
(I know that cheer can be written as hearten)
'perceive a number' is the wordplay.
'perceive' becomes 'hear' (hearing is a kind of perceiving).
'a number' becomes 'ten' (ten is a number).
'hear'+'ten'='HEARTEN'
(Other definitions for hearten that I've seen before include "Encourage in Teheran" , "Inspire anew" , "Comfort" , "giving cheer" , "Be encouraged" .)