Affection's gone west -- moving out in progress (9)
I believe the answer is:
evolution
'progress' is the definition.
(thesaurus)
'affection's gone west moving out in' is the wordplay.
'affection' becomes 'love' (I've seen this before).
'gone west' shows that the letters should be reversed in order (letters written from \'east to west\').
'moving' indicates anagramming the letters.
'love' back-to-front is 'evol'.
'out'+'in'='outin'
'outin' anagrammed gives 'ution'.
'evol'+'ution'='EVOLUTION'
(Other definitions for evolution that I've seen before include "it's a gradual process" , "Slow change" , "Gradual change" , "Darwin's theory" , "Development - progress" .)