Why don't we put up a stone monument? (5)
I believe the answer is:
stela
'stone monument?' is the definition.
(I've seen this in another clue)
'why don't we put up a' is the wordplay.
'why don't we' becomes 'lets' (I've seen this in another clue).
'put up' is a reversal indicator.
'lets' back-to-front is 'stel'.
'stel'+'a'='STELA'
(Other definitions for stela that I've seen before include "Upright stone slab" , "Inscribed column" , "Stone tablet" , "Standing stone" , "People having digs" .)