Admit cipher is broken by one crew regularly (7)
I believe the answer is:
concede
'admit' is the definition.
(I know that concede is a more specific form of admit)
'cipher is broken by one crew regularly' is the wordplay.
'cipher' becomes 'code' (synonyms).
'is broken by' indicates putting letters inside (inserted letters break into the word).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'one'+'crew'='onecrew'
The alternate letters of 'onecrew' are 'nce'.
'code' placed around 'nce' is 'CONCEDE'.
(Other definitions for concede that I've seen before include "Yield; grant" , "Admit; yield" , "Acknowledge defeat" , "Surrender, yield" , "Admit; surrender" .)