Follow unseen, mostly in disguise (5)
I believe the answer is:
ensue
'follow' is the definition.
(thesaurus)
'unseen mostly in disguise' is the wordplay.
'mostly' means to remove the last letter.
'in disguise' indicates anagramming the letters.
'unseen' with its final letter removed is 'unsee'.
'unsee' with letters rearranged gives 'ENSUE'.
(Other definitions for ensue that I've seen before include "Follow as a consequence" , "Happen - to come later" , "Follow afterwards" , "Proceed" , "Follow later" .)