Phantom jockey crept inside stall regularly (8)
I believe the answer is:
spectral
'phantom' is the definition.
(I've seen this in another clue)
'jockey crept inside stall regularly' is the wordplay.
'jockey' is an anagram indicator (the letters scramble or jockey around).
'inside' is an insertion indicator.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'stall' are 'sal'.
'crept' anagrammed gives 'pectr'.
'pectr' put into 'sal' is 'SPECTRAL'.
(Other definitions for spectral that I've seen before include "Like a colourful band" , "Rap Celts for being ghostly" , "Of or like a ghost" , "Ghostly; of bands of colour" , "Ghastly" .)