One month is a long time in pictures (6)
I believe the answer is:
images
'pictures' is the definition.
(I know this)
'one month is a long time' is the wordplay.
'one' becomes 'I' (Roman numeral).
'month' becomes 'M' (standard abbreviation).
'is a long time' becomes 'ages' (I've seen this before).
'i'+'m'+'ages'='IMAGES'
'in' acts as a link.
(Other definitions for images that I've seen before include "Visual counterparts in lens or mirror" , "Likenesses or mental pictures" , "Likenesses, representations" , "Visual or mental representations" , "Photos or likenesses" .)