Fall back as regiment begins to go by (7)
I believe the answer is:
relapse
'fall back' is the definition.
(synonyms)
'regiment begins to go by' is the wordplay.
'begins to' indicates taking the first letters (I've seen 'beginning to' mean this).
'go by' becomes 'elapse' (synonyms).
The initial letter of 'regiment' is 'r'.
'r'+'elapse'='RELAPSE'
'as' acts as a link.
(Other definitions for relapse that I've seen before include "Fall back (into ill health)" , "Fail to maintain progress" , "Backsliding" , "Repeals (anag.)" , "Deterioration after period of improvement" .)