Flowing water I have discovered in middle of burrow (5)
I believe the answer is:
river
'flowing water' is the definition.
(I've seen this before)
'i have discovered in middle of burrow' is the wordplay.
'i have' becomes 'ive'.
'discovered in' indicates putting letters inside.
'middle of' means to look at the middle letters.
The middle of 'burrow' is 'rr'.
'ive' placed into 'rr' is 'RIVER'.
(Other definitions for river that I've seen before include "Dee, Exe or Wye, say" , "Waterway" , "Thames is one" , "Eg, Thames or Nile" , "Large flow of liquid" .)