I had trouble ultimately in inadequate form of illumination (9)
I believe the answer is:
sidelight
'form of illumination' is the definition.
'i had trouble ultimately in inadequate' is the wordplay.
'i had' becomes 'id' (I'd).
'ultimately' indicates one should take the final letters.
'in' means one lot of letters goes inside another.
'inadequate' becomes 'slight'.
The last letter of 'trouble' is 'e'.
'id'+'e'='ide'
'ide' placed into 'slight' is 'SIDELIGHT'.
(Other definitions for sidelight that I've seen before include "Visible warning" , "Window" , "One shines" , "Car part" , "Additional or incidental information" .)