Extremely furtive row of cats (6)
I believe the answer is:
feline
'of cats' is the definition.
(I've seen this before)
'extremely furtive row' is the wordplay.
'extremely' means to remove the middle letters.
'row' becomes 'line' (I've seen this before).
'furtive' with its middle taken out is 'fe'.
'fe'+'line'='FELINE'
(Other definitions for feline that I've seen before include "Relating to cats" , "16 [TOM] perhaps" , "Like a cat" , "Mog" , "Pertaining to cats" .)