Contest to go across school with three legs? (8)

I believe the answer is:

tripodal

'with three legs?' is the definition.
(tripodal means having three legs)

'contest to go across school' is the wordplay.
'contest' becomes 'trial' (trial can mean a sports contest).
'to go across' is an insertion indicator (some letters go across or around others).
'school' becomes 'pod' (term for a group of marine mammals).
'trial' going around 'pod' is 'TRIPODAL'.

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app