Start to reduce crush — please, fall back! (7)
I believe the answer is:
relapse
'fall back' is the definition.
(synonyms)
'start to reduce crush please' is the wordplay.
'start to' suggests taking the first letters.
'crush' indicates an anagram (I've seen 'crushed' mean this).
The first letter of 'reduce' is 'r'.
'please' is an anagram of 'elapse'.
'r'+'elapse'='RELAPSE'
(Other definitions for relapse that I've seen before include "Downturn in health (after improvement)" , "Fall ill again after rallying" , "Fall back like leapers" , "Sink back into illness" , "Backsliding" .)