Former adjustment to rest period (9)
I believe the answer is:
erstwhile
'former' is the definition.
(I know that former can be written as erstwhile)
'adjustment to rest period' is the wordplay.
'adjustment to' is an anagram indicator.
'period' becomes 'while' (while is a kind of period).
'rest' is an anagram of 'erst'.
'erst'+'while'='ERSTWHILE'
(Other definitions for erstwhile that I've seen before include "Formerly" , "Former - list where (anag)" , "In the past" , "Former, previous" , "Quondam" .)