After time register one living under bridge? (5)
I believe the answer is:
troll
'one living under bridge?' is the definition.
(as in the fairy-tale creature)
'after time register' is the wordplay.
'after' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'register' becomes 'roll' (list of names).
't' put next to 'roll' is 'TROLL'.
(Other definitions for troll that I've seen before include "Supernatural creature of Scandinavian folklore" , "Fish with a moving bait" , "Fairytale monster" , "Ugly character from Norse legend" , "Possibly a monster" .)