Sit astride rider's seat holding both ends of tether (8)
I believe the answer is:
straddle
'sit astride' is the definition.
(I've seen this before)
'rider's seat holding both ends of tether' is the wordplay.
'rider's seat' becomes 'saddle' (I've seen this before).
'holding' means one lot of letters goes inside another.
'both' becomes 't' (I've seen this in other clues).
'ends of' indicates one should take the final letters (I've seen 'end of' mean this).
The final letter of 'tether' is 'r'.
't'+'r'='tr'
'saddle' enclosing 'tr' is 'STRADDLE'.
(Other definitions for straddle that I've seen before include "jumping technique" , "Bestride" , "Stand with legs either side of" , "Stretch legs across" , "Sit astride" .)