One in the act coming back to complain (4)
I believe the answer is:
wail
'complain' is the definition.
(I've seen this in another clue)
'one in the act coming back' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'the act' becomes 'law' (law is a kind of act).
'coming back' is a reversal indicator.
'i' placed inside 'law' is 'liaw'.
'liaw' back-to-front is 'WAIL'.
'to' acts as a link.
(Other definitions for wail that I've seen before include "Sorrowful cry" , "Prolonged cry" , "Cry of grief" , "(Make a) plaintive cry" , "Lament audibly" .)