Grow profusely in enclosure -- impressively large display (3,4)
I believe the answer is:
run riot
'grow profusely' is the definition.
(I've seen this before)
'enclosure impressively large display' is the wordplay.
'enclosure' becomes 'run' (animal enclosure eg a chicken run).
'impressively large display' becomes 'riot' (eg a riot of colour).
'run'+'riot'='RUN RIOT'
'in' is the link.
(Other definitions for run riot that I've seen before include "Go wild" , "Act wantonly, without restraint" , "Grow profusely (of plants)" , "Behave in unruly way like mob" , "Spread uncontrollably" .)