Please do arithmetic in valley (7)
I believe the answer is:
gladden
'please' is the definition.
(I've seen this before)
'do arithmetic in valley' is the wordplay.
'do arithmetic' becomes 'add' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'valley' becomes 'glen' (glen is a kind of valley).
'add' placed inside 'glen' is 'GLADDEN'.
(Other definitions for gladden that I've seen before include "Give pleasure to" , "Hearten" , "Lift (the heart)" , "Make joyful" , "Dangled (anag) - cheer" .)