Pentagon in front of stone a fair attraction (6)
I believe the answer is:
dodgem
'a fair attraction' is the definition.
(dodgems are a fairground ride)
'pentagon in front of stone' is the wordplay.
'pentagon' becomes 'dod' (US Department of Defense, based in the Pentagon).
'in front of' is a charade indicator (letters next to each other).
'stone' becomes 'gem' (I've seen this in other clues).
'dod'+'gem'='DODGEM'
(Other definitions for dodgem that I've seen before include "Electric car" , "ride - with 14a" , "Car used in fairground" , "Fairground amusement vehicle" , "Bumper car" .)