What might measure article on the fourth dimension? (9)
I believe the answer is:
timepiece
'what might measure' is the definition.
(a timepiece measures time)
'article on the fourth dimension?' is the wordplay.
'article' becomes 'piece' (piece is a kind of article).
'on' means one lot of letters go next to another.
'the fourth dimension?' becomes 'time' (time is sometimes considered the fourth dimension).
'piece' after 'time' is 'TIMEPIECE'.
(Other definitions for timepiece that I've seen before include "Clock, for example" , "Watch, say" , "alarm, perhaps" , "Maybe watch" , "non-striker?" .)