Good walker who's rather slow gets better (7)
I believe the answer is:
gambler
'better' is the definition.
(a gambler is someone who bets)
'good walker who's rather slow' is the wordplay.
'good' becomes 'g' (abbreviation).
'walker who's rather slow' becomes 'ambler' (I've seen this before).
'g'+'ambler'='GAMBLER'
'gets' acts as a link.
(Other definitions for gambler that I've seen before include "Risk taker" , "one who takes chances!" , "I bet?" , "Better" , "One who bets" .)