Expansion keeping Times in rear (6)
I believe the answer is:
bottom
'rear' is the definition.
(I know that rear can be written as bottom)
'expansion keeping times' is the wordplay.
'expansion' becomes 'boom' (period of economic growth).
'keeping' indicates putting letters inside.
'times' becomes 'tt' (two abbreviations for time).
'boom' placed around 'tt' is 'BOTTOM'.
'in' acts as a link.
(Other definitions for bottom that I've seen before include "Lowest point - weaver" , "Athenian weaver (AMND)" , "foot" , "A Midsummer-Night's Dream character" , "Last" .)