Principal record husband set in motorists' club (5)
I believe the answer is:
alpha
'principal' is the definition.
(both can mean the most important or first)
'record husband set in motorists' club' is the wordplay.
'record' becomes 'LP' (abbreviation for long play record).
'husband' becomes 'h'.
'set in' is an insertion indicator.
'motorists' club' becomes 'AA' (the AA, Automobile Association).
'lp'+'h'='lph'
'lph' placed within 'aa' is 'ALPHA'.
(Other definitions for alpha that I've seen before include "leader in Athens, originally" , "Greek vowel" , "star of utmost brilliance" , "very good rating" , "starter" .)