Measurement encompassing daughter alongside (5)
I believe the answer is:
Here is my best explanation:
'measurement' is the definition.
(width is a kind of measurement)
'encompassing daughter alongside' is the wordplay.
'encompassing' is an insertion indicator.
'daughter' becomes 'd'.
'alongside' becomes 'with' (I've seen this before).
'd' placed inside 'with' is 'WIDTH'.