It's a tall order requiring to be posh (4-5)
I believe the answer is:
high-class
'to be posh' is the definition.
(I've seen this before)
'it's a tall order' is the wordplay.
'it's a tall' becomes 'high' (I've seen this before).
'order' becomes 'class' (I've seen this in another clue).
'high'+'class'='HIGH-CLASS'
'requiring' is the link.
(Other definitions for high-class that I've seen before include "Posh" , "Superior" .)