Hardship of most of the outfit from regiment (6)
I believe the answer is:
Here is my best explanation:
'hardship of most' is the definition.
'rigour' can be an answer for 'hardship' ('rigour' can be a synonym of 'hardship'). I am not sure about the 'of most' bit.
'the outfit from regiment' is the wordplay.
'the outfit' becomes 'rig' ('rig' can be a synonym of 'outfit').
'from' is a charade indicator (letters next to each other).
'regiment' becomes 'our' (I can't justify this - if you can you should believe this answer much more).
'of' is the link.
(Other definitions for rigour that I've seen before include "Extreme strictness", "Severity or stringency", "Strictness - austerity", "Trial", "Quality of being extremely thorough and careful", "Severity or strictness, perhaps of the law", "Preciseness - severity", "Thoroughness", "Harshness, severity" and "Stiffness or strictness".)