Odds on expert having room (5)
I believe the answer is:
space
'room' is the definition.
(I've seen this before)
'odds on expert' is the wordplay.
'odds' becomes 'sp' (betting abbreviation for starting price).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'expert' becomes 'ace' (ace can informally mean very good).
'sp'+'ace'='SPACE'
'having' acts as a link.
(Other definitions for space that I've seen before include "Emptiness" , "Cosmos" , "Unfilled region" , "Gap between words" , ""The final frontier"" .)