Housing engineer left immediately to build extension? (8)
I believe the answer is:
lengthen
'build extension?' is the definition.
'housing engineer left immediately' is the wordplay.
'housing' is an insertion indicator.
'engineer' becomes 'eng'.
'left' becomes 'l' (common abbreviation).
'immediately' becomes 'then'.
'l'+'then'='lthen'
'eng' put into 'lthen' is 'LENGTHEN'.
'to' acts as a link.
(Other definitions for lengthen that I've seen before include "increase in duration" , "Extend or draw out" , "Prolong" , "Stretch" , "Spin out" .)