What can increase power of fantastic horse (12)
I believe the answer is:
supercharger
'what can increase power' is the definition.
(engine part)
'fantastic horse' is the wordplay.
'fantastic' becomes 'super' (I've seen this before).
'horse' becomes 'charger' (charger is a type of horse).
'super'+'charger'='SUPERCHARGER'
'of' is the link.
(Other definitions for supercharger that I've seen before include "that makes for efficient transport" , "Engine efficiency improver" , "Device improving an engine's efficiency" .)