Vague, relative to a regular start (7)
I believe the answer is:
unclear
'vague' is the definition.
(similar in meaning)
'relative to a regular start' is the wordplay.
'relative' becomes 'uncle' (uncle is a kind of relative).
'to' says to put letters next to each other.
'start' says to take the initial letters.
The initial letter of 'regular' is 'r'.
'uncle'+'a'+'r'='UNCLEAR'
(Other definitions for unclear that I've seen before include "Not easy to see or understand" , "Not certain" , "Imprecise, hazy" , "Cloudy or opaque" , "Nebulous" .)