A popular action to take, unquestionably (6)
I believe the answer is:
indeed
'unquestionably' is the definition.
(I've seen this before)
'a popular action' is the wordplay.
'a popular' becomes 'in' (something that's 'in' is fashionable).
'action' becomes 'deed' (deed is a kind of action).
'in'+'deed'='INDEED'
'to take' acts as a link.
(Other definitions for indeed that I've seen before include "Truly or admittedly" , "Really, without doubt" , "Word expressing surprise, disbelief, etc" , "Unquestionably" , "Truthfully, really" .)