Nick -- name involved in downfall? (3,2)
I believe the answer is:
run in
'nick' is the definition.
'name involved in downfall?' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'involved in' is an insertion indicator.
'downfall?' becomes 'ruin' (I've seen this before**).
'n' put within 'ruin' is 'RUN-IN'.
(Other definitions for run in that I've seen before include "final stages of race" , "Carefully bring to full working order - arrest" , "Final stage" , "Disagreement, fight (colloq.)" , "Pick up" .)