Cheers up on course to get fit (6)
I believe the answer is:
attack
'fit' is the definition.
(I've seen this before)
'cheers up on course' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'on' means one lot of letters go next to another.
'course' becomes 'tack' (I've seen this before).
'ta' backwards is 'at'.
'at'+'tack'='ATTACK'
'to get' is the link.
(Other definitions for attack that I've seen before include "Fiercely criticise" , "Offensive action" , "Pitch into" , "Offensive move" , "An assault or onslaught" .)