Helped one in, dead beat (5)
I believe the answer is:
aided
'helped' is the definition.
('aid' can be a synonym of 'help')
'one in dead beat' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'beat' is an anagram indicator.
'dead' anagrammed gives 'aded'.
'i' put into 'aded' is 'AIDED'.
(Other definitions for aided that I've seen before include "given assistance" , "'Assisted, helped (5)'" , "not under their own steam?" .)