Troops surrender in a slump (9)
I believe the answer is:
'a slump' is the definition.
(a recession is an economic slump)
'troops surrender' is the wordplay.
'troops' becomes 'RE' (Royal Engineers army regiment).
'surrender' becomes 'cession' (cession is surrendering or ceding something).
'in' acts as a link.
(Other definitions for recession that I've seen before include "Down-turn" , "when times are hard" , "falling back" , "Widespread decline in employment and trade" , "Period of temporary economic decline" .)