Pins up a favourite piece of information (7)
I believe the answer is:
snippet
'piece of information' is the definition.
(I've seen this before)
'pins up a favourite' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'a favourite' becomes 'pet' (synonyms).
'pins' in reverse letter order is 'snip'.
'snip'+'pet'='SNIPPET'
(Other definitions for snippet that I've seen before include "Small fragment" , "Small extract" , "Small scrap" , "Brief extract" , "excerpt of literature?" .)