Where it should be popular, pose for all to see (2,4)
I believe the answer is:
in situ
'where' is the definition.
Both the definition and answer are adverbs. Maybe you can see an association between them that I don't see?
'popular pose for all to see' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'pose' becomes 'sit' (synonyms).
'for all to see' becomes 'u' (short for Universal film certificate).
'in'+'sit'+'u'='IN-SITU'
'it should be' is the link.
Can you help me to learn more?
(Other definitions for in situ that I've seen before include "Appropriately located" , "Correctly placed" , "Latin meaning 'on site'" , "Being in the original position, not having been moved" , "At the appropriate location" .)