Plan great changes in animal enclosure (8)
I believe the answer is:
strategy
'plan' is the definition.
(strategy is a kind of plan)
'great changes in animal enclosure' is the wordplay.
'changes' indicates an anagram.
'in' indicates putting letters inside.
'animal enclosure' becomes 'sty' (pig sty).
'great' with letters rearranged gives 'rateg'.
'rateg' going into 'sty' is 'STRATEGY'.
(Other definitions for strategy that I've seen before include "Overall plan for campaign" , "Grand design" , "manoeuvres" , "Planning and direction of campaign" , "Get satyr to formulate tactics" .)