Start doing gags about Charlie being seen at Cheltenham? (4,6)
I believe the answer is:
jump jockey
'seen at cheltenham?' is the definition.
(Cheltenham is a racecourse)
'start doing gags about charlie' is the wordplay.
'start' becomes 'jump' (jumping is a kind of starting).
'doing gags' becomes 'jokey'.
'about' is an insertion indicator.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'jokey' enclosing 'c' is 'jockey'.
'jump'+'jockey'='JUMP JOCKEY'
'being' acts as a link.
(Other definitions for jump jockey that I've seen before include "Rider over the sticks" , "National competitor" .)