Part of six-pack: Mark carrying can out (6)
I believe the answer is:
abloom
'out' is the definition.
(I've seen this before)
'part of six-pack mark carrying can' is the wordplay.
'part of six pack' becomes 'ab' (short for abdominal muscle).
'mark' becomes 'm'.
'carrying' says to put letters next to each other (in a down clue, the bottom letters carry others).
'can' becomes 'loo' (both can informally mean toilet).
'm' put after 'loo' is 'loom'.
'ab'+'loom'='ABLOOM'
(Other definitions for abloom that I've seen before include "Healthy" , "full of colour" , "Flowering" , "Flourishing" , "In flower" .)