Stylish horse guards at military base (12)
I believe the answer is:
installation
'military base' is the definition.
(I've seen this before)
'stylish horse guards at' is the wordplay.
'stylish' becomes 'in'.
'horse' becomes 'stallion' (I've seen this before).
'guards' is an insertion indicator (inserted letters are guarded).
'stallion' going around 'at' is 'stallation'.
'in'+'stallation'='INSTALLATION'
(Other definitions for installation that I've seen before include "establishment" , "The making of equipment ready for use" , "Apparatus positioned" , "Positioning" , "Art exhibit in gallery" .)