Eat in less irregularly — that’s vital! (9)
I believe the answer is:
essential
'vital' is the definition.
(similar in meaning)
'eat in less irregularly' is the wordplay.
'irregularly' is an anagram indicator.
'eat'+'in'+'less'='eatinless'
'eatinless' is an anagram of 'ESSENTIAL'.
'that's' acts as a link.
(Other definitions for essential that I've seen before include "Crucial" , "must-have item" , "Vitally important" , "Key" , "Type of oil" .)