Prehistoric monument seen on the ground, inspiring gasps initially (10)
I believe the answer is:
stonehenge
'prehistoric monument' is the definition.
'seen on the ground inspiring gasps initially' is the wordplay.
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'inspiring' indicates putting letters inside.
'initially' says to take the initial letters.
The first letter of 'gasps' is 'g'.
'seen'+'on'+'the'='seenonthe'
'seenonthe' with letters rearranged gives 'stonehene'.
'stonehene' placed around 'g' is 'STONEHENGE'.
(Other definitions for stonehenge that I've seen before include "Megalithic monument on Salisbury Plain" , "Wiltshire attraction" , "Prehistoric ruin, Salisbury Plain" , "Wiltshire prehistoric monument" , "Druid comes here" .)