Roughly one drop of rain in paltry fountain (6,4)
I believe the answer is:
pretty well
'roughly' is the definition.
'one drop of rain in paltry fountain' is the wordplay.
'one drop of' indicates taking the first letters.
'in' indicates putting letters inside.
'paltry' becomes 'petty'.
'fountain' becomes 'well'.
The initial letter of 'rain' is 'r'.
'r' put inside 'petty' is 'pretty'.
'pretty'+'well'='PRETTY WELL'
(Other definitions for pretty well that I've seen before include "Practically" , "Close to" , "Aim of dressing in Derbyshire" , "Sort of" , "More or less" .)