Separate legs, taking seat across sides of tractor (8)
I believe the answer is:
straddle
'separate legs' is the definition.
(to straddle is put one's legs around something)
'seat across sides of tractor' is the wordplay.
'seat' becomes 'saddle' (saddle is a kind of seat).
'across' means one lot of letters goes inside another.
'sides of' means to remove the middle letters (only the letters on the sides remain).
'tractor' with its centre taken out is 'tr'.
'saddle' placed around 'tr' is 'STRADDLE'.
'taking' acts as a link.
(Other definitions for straddle that I've seen before include "Gymnastic exercise with a leg on either side of parallel bars" , "Step across" , "To have a foot in both camps" , "jumping technique" , "Led darts (anag)" .)