Admit couple regularly will get upset, it's clear (5)
I believe the answer is:
lucid
'clear' is the definition.
(similar in meaning)
'admit couple regularly will get upset' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'will get upset' shows that the letters should be reversed in order (in a down clue, letters go up).
'admit'+'couple'='admitcouple'
The alternate letters of 'admitcouple' are 'dicul'.
'dicul' in reverse letter order is 'LUCID'.
'it's' acts as a link.
(Other definitions for lucid that I've seen before include "Clear; making sense" , "completely clear" , "Transparent" , "Able to think clearly" , "Clear; bright" .)