Blunder among reserves in ground (7)
I believe the answer is:
terrain
'ground' is the definition.
(thesaurus)
'blunder among reserves in' is the wordplay.
'blunder' becomes 'err' (I've seen this before).
'among' means one lot of letters goes inside another.
'reserves' becomes 'ta' (I've seen this in other clues).
'err' put into 'ta' is 'terra'.
'terra'+'in'='TERRAIN'
(Other definitions for terrain that I've seen before include "Tract of land with particular character" , "Tract of land of a specific character" , "Tract of ground with specific characteristics" , "Piece of ground" , "Physical feature of a stretch of ground" .)