K pop, mostly, is a must when dancing (9)
I believe the answer is:
potassium
'k' is the definition.
(synonyms)
'pop mostly is a must when dancing' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'when dancing' indicates anagramming the letters (letters dance around).
'pop' with its last letter removed is 'po'.
'is'+'a'+'must'='isamust'
'isamust' with letters rearranged gives 'tassium'.
'po'+'tassium'='POTASSIUM'
(Other definitions for potassium that I've seen before include "Element K" , "... is its atomic number: symbol K" , "Soft metallic element" , "Silvery metallic element" , "Reactive metal element" .)