Shrub, say, mostly seen in plot (4)
I believe the answer is:
plan
'plot' is the definition.
(thesaurus)
'shrub say mostly' is the wordplay.
'shrub say' becomes 'plant' (shrub is a kind of plant).
'mostly' means to remove the last letter.
'plant' with its final letter taken away is 'PLAN'.
'seen in' acts as a link.
(Other definitions for plan that I've seen before include "Project" , "Scheme to achieve something" , "Stratagem" , "Diagram; design" , "Programme" .)