Pins up favourite piece of information (7)
I believe the answer is:
snippet
'piece of information' is the definition.
(I've seen this before)
'pins up favourite' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'pins' back-to-front is 'snip'.
'snip'+'pet'='SNIPPET'
(Other definitions for snippet that I've seen before include "Brief extract" , "excerpt of literature?" , "A small piece cut off" , "piece of information" , "Newspaper extract" .)