Ruminating (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ruminating</span>' is the definition.<br/ ><span class="explanation-format__explanation">('muse' can be a synonym of 'ruminate')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for musing that I've seen before include "reflection" , "Meditating" , "Reflecting in silence" , "Pondering, thinking" , "Cogitation" .)