Erection of some flats, ninety put in (6)
I believe the answer is:
instal
'put in' is the definition.
(I know that put in can be written as instal)
'erection of some flats ninety' is the wordplay.
'erection' is a reversal indicator (I've seen 'erection of' mean this (in a down clue going up is a reversal)).
'of some' says the answer is hidden in the clue (I've seen 'some of' mean this).
'latsni' is hidden in the letters of 'flats ninety'.
'latsni' backwards is 'INSTAL'.
(Other definitions for instal that I've seen before include "Invest in office" , "Place in position, ready for use" , "Put in place and make ready for use" , "institute" , "Set up ready for use" .)