Happiness is found in March (8)
I believe the answer is:
paradise
'happiness' is the definition.
(I've seen this before)
'is found in march' is the wordplay.
'found in' means one lot of letters goes inside another.
'march' becomes 'parade' (parading is a kind of marching).
'is' put within 'parade' is 'PARADISE'.
(Other definitions for paradise that I've seen before include "Idyllic place" , "Shangri-La?" , "Heaven; Eden" , "Eden; ideal place" , "Sensual garden" .)