Triathletes initially walk unevenly? Nonsense (7)
I believe the answer is:
twaddle
'nonsense' is the definition.
(informal term for nonsense or rubbish)
'triathletes initially walk unevenly?' is the wordplay.
'initially' suggests taking the first letters.
'walk unevenly?' becomes 'waddle' (to waddle is to walk clumsily).
The initial letter of 'triathletes' is 't'.
't'+'waddle'='TWADDLE'
(Other definitions for twaddle that I've seen before include "Foolish talk" , "Foolish speech" , "Rubbish" , "Moonshine" , "Balderdash" .)