Period when expecting a lot of frost in high street (9)
I believe the answer is:
trimester
'period when expecting' is the definition.
(period of time when expecting a baby)
'a lot of frost in high street' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'frost' becomes 'rime' (rime is frost formed by freezing water vapour).
'in' indicates putting letters inside.
'high' indicates an anagram (can mean drunk or on drugs and hence disordered).
'rime' with its last letter removed is 'rim'.
'street' is an anagram of 'tester'.
'rim' placed within 'tester' is 'TRIMESTER'.
(Other definitions for trimester that I've seen before include "US academic term" , "Period of three months" , "part of college year" , "American term" , "academic session" .)