Instrumental recording, set in regular three-four, on Light originally (7)
I believe the answer is:
helpful
'instrumental' is the definition.
(similar in meaning)
'recording set in regular three-four on light originally' is the wordplay.
'recording' becomes 'lp' (LP is an example).
'set in' means one lot of letters goes inside another.
'regular' means one should take alternating letters.
'on' is a charade indicator (letters next to each other).
'originally' says to take the initial letters.
The initial letter of 'light' is 'l'.
'three'+'four'='threefour'
The alternate letters of 'threefour' are 'hefu'.
'lp' placed inside 'hefu' is 'helpfu'.
'helpfu'+'l'='HELPFUL'
(Other definitions for helpful that I've seen before include "Willing to be of assistance" , "Of service or assistance" , "Obliging" , "Co-operative" , "Of assistance, supportive" .)