It can raise a smile — even when blue? (6)
I believe the answer is:
cheese
'it can raise a smile' is the definition.
(people having their photo taken say 'cheese')
I can't explain the remainder of the clue.
(Other definitions for cheese that I've seen before include "Big one's important" , "Lancashire, perhaps" , "Eg Edam" , "Nautical cap worn without badge" , "Eg, cheddar" .)