Afterthought about a source of spring water (3)
I believe the answer is:
spa
'source of spring water' is the definition.
(a spa is a source of water)
'afterthought about a' is the wordplay.
'afterthought' becomes 'ps' (ps stands for postscript).
'about' is a reversal indicator.
'ps' backwards is 'sp'.
'sp'+'a'='SPA'
(Other definitions for spa that I've seen before include "Health resort at a mineral spring" , "Hydromassage facility" , "Watering-place" , "Water-cure town" , "well" .)