Annual interest, one pound! (5)
I believe the answer is:
april
'annual interest' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'one pound' is the wordplay.
'one' becomes 'apri' (I am not sure about this - if you are sure you should believe this answer much more).
'pound' becomes 'L' (the pound sign is based on an L).
'apri'+'l'='APRIL'
Can you help me to learn more?
(Other definitions for april that I've seen before include "Spring month" , "''... is the cruellest month'' (TS Eliot)" , "Traditionally showery month" , "''All in an ... evening / ... airs were abroad'' (by Katherine Tynan)" , "'All in an ... evening' (Katharine Tynan)" .)