Place to secure almost all hash and E, then start using gradually (5,2)
I believe the answer is:
'start using gradually' is the definition.
(to phase in is to gradually introduce)
'place to secure almost all hash and e' is the wordplay.
'place' becomes 'pin'.
'to secure' indicates putting letters inside.
'almost all' means to remove the last letter.
'and' means one lot of letters go next to another.
'hash' with its final letter taken off is 'has'.
'pin' going around 'hase' is 'PHASE IN'.
'then' acts as a link.
(Other definitions for phase in that I've seen before include "Introduce by stages", "introduce slowly", "Introduce gradually".)