Small window in room secured by dynamic person (6)
I believe the answer is:
dormer
'small window' is the definition.
(type of window)
'room secured by dynamic person' is the wordplay.
'room' becomes 'rm' (abbreviation).
'secured by' is an insertion indicator (inserted letters are held or secured).
'dynamic person' becomes 'doer' (someone who does things).
'rm' put inside 'doer' is 'DORMER'.
'in' is the link.
(Other definitions for dormer that I've seen before include "Window under gable on sloping roof" , "Type of window set in sloping roof" , "Window projecting vertically from a sloping roof" , "Window fitted vertically onto a sloping roof." , "Kind of window under gable" .)