University dons occasionally too cold in architectural openings (5)
I believe the answer is:
oculi
'architectural openings' is the definition.
(I've seen this in another clue)
'university dons occasionally too cold in' is the wordplay.
'university' becomes 'U' (abbreviation for university).
'dons' means one lot of letters goes inside another (don means to wear or put on).
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'too'+'cold'+'in'='toocoldin'
The alternating letters of 'toocoldin' are 'ocli'.
'u' inserted into 'ocli' is 'OCULI'.
(Other definitions for oculi that I've seen before include "Eyes; eye-like opening in architecture, like a round window" , "Circular windows" .)