Cheers up, on course to get fit (6)
I believe the answer is:
'fit' is the definition.
'cheers up on course' is the wordplay.
'cheers up' becomes 'at' (I've seen this in other clues).
'on' says to put letters next to each other.
'course' becomes 'tack' (I've seen this before**).
'to get' acts as a link.
(Other definitions for attack that I've seen before include "Fiercely criticise" , "Offensive action" , "Pitch into" , "Offensive move" , "An assault or onslaught" .)