One then catching a cold - rough! (7)
I believe the answer is:
inexact
'rough' is the definition.
(similar in meaning)
'one then catching a cold' is the wordplay.
'one' becomes 'I' (Roman numeral).
'then' becomes 'next' (I can't justify this - if you can you should believe this answer much more).
'catching' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'a'+'c'='ac'
'next' placed around 'ac' is 'nexact'.
'i'+'nexact'='INEXACT'
Can you help me to learn more?
(Other definitions for inexact that I've seen before include "not wholly Right" , "Fuzzy, muddled" , "Fuzzy - untrue" , "Vague" , "Woolly" .)