Great flow under ground (9)
I believe the answer is:
wonderful
'great' is the definition.
(I've seen this before)
'flow under ground' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'flow'+'under'='flowunder'
'flowunder' anagrammed gives 'WONDERFUL'.
(Other definitions for wonderful that I've seen before include "Extraordinary" , "Marvellous" , "Superb" , "Fantastic, terrific" , "Great" .)