On a seat grasping Trigger's flanks? (9)
I believe the answer is:
astraddle
'seat grasping trigger's flanks?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'on a' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
The remaining letters 'straddle' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for astraddle that I've seen before include "Across" , "With a leg on either side of (something)" , "How the jockey wants to be" , "with legs wide apart" .)