Spring appears, finally, every year (3)
I believe the answer is:
Here is my best explanation:
'spring' is the definition.
'appears finally every year' is the wordplay.
'finally' suggests the final letters.
'every year' becomes 'pa' (per annum).
The last letter of 'appears' is 's'.
(Other definitions for spa that I've seen before include "Bath - mineral water spring", "Health and beauty treatment establishment", "here for healthy holiday", "E.g. Harrogate", "Spring", "hydro", "eg Bath", "Resort with mineral water spring", "town in Belgium" and "Watering-place".)