A source of light forces change outside (8)
I believe the answer is:
alfresco
'outside' is the definition.
(I've seen this before)
'a source of light forces change' is the wordplay.
'source of' says to take the initial letters.
'change' indicates anagramming the letters.
The first letter of 'light' is 'l'.
'forces' anagrammed gives 'fresco'.
'a'+'l'+'fresco'='ALFRESCO'
(Other definitions for alfresco that I've seen before include "out of the house" , "exposed to the fresh air" , "In the open air" , "Located outdoors" , "Out of doors" .)