Belief in brief (5)
(Other definitions for ism that I've seen before include "Ideological movement" , "Any doctrine or theory" , "Informally, any doctrine or practice" , "Stage performance of singing" , "Suffix denoting doctrine or practice" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">belief in brief</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for credo that I've seen before include "Formal statement of belief" , "music in church" , "Doctrine or belief" , "Guiding beliefs" , "A statement of one's beliefs" .)