Fall into part of the main current (5)
I believe the answer is:
incur
'fall into' is the definition.
(I've seen this before)
'part of the main current' is the wordplay.
'part of' indicates a hidden word.
'INCUR' is hidden within 'main current'.
(Other definitions for incur that I've seen before include "To become liable to, suffer" , "Invite, bring about" , "Meet" , "Provoke" , "Run up" .)