In grassland antelope go slowly (7)
I believe the answer is:
andante
'slowly' is the definition.
(musical term)
'in grassland antelope go' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ANDANTE' can be found hidden inside 'grassland antelope go'.
(Other definitions for andante that I've seen before include "Moderately slow movement" , "Fairly slow (mus.)" , "deliberately played" , "Musically, in moderately slow time" , "Unhurried pace" .)