Plan great changes in farm building (8)
I believe the answer is:
strategy
'plan' is the definition.
(strategy is a kind of plan)
'great changes in farm building' is the wordplay.
'changes' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'farm building' becomes 'sty'.
'great' anagrammed gives 'rateg'.
'rateg' inserted inside 'sty' is 'STRATEGY'.
(Other definitions for strategy that I've seen before include "Game plan" , "Plan or policy to achieve something" , "Approach" , "Get satyr to formulate tactics" , "Overall plan for campaign" .)