Appropriate picture - source of light in heavens (6)
I believe the answer is:
seemly
'appropriate' is the definition.
(I've seen this before)
'picture source of light in heavens' is the wordplay.
'picture' becomes 'see' (synonyms).
'source of' says to take the initial letters.
'in' means one lot of letters goes inside another.
'heavens' becomes 'my' (both are expressions of shock or disbelief).
The first letter of 'light' is 'l'.
'l' placed within 'my' is 'mly'.
'see'+'mly'='SEEMLY'
(Other definitions for seemly that I've seen before include "'Decorous, dignified (6)'" , "Part of the foot" , "Appropriate" , "Decorous, in good taste" , "Conforming to accepted notions of propriety" .)