It's clear 50 are in acute discomfort (5)
I believe the answer is:
plain
'it's clear' is the definition.
(similar in meaning)
'50 are in acute discomfort' is the wordplay.
'50' becomes 'l' (Roman numeral).
'are in' indicates putting letters inside.
'acute discomfort' becomes 'pain' (I've seen this in another clue. I am not sure about the 'acute' bit.).
'l' placed into 'pain' is 'PLAIN'.
Can you help me to learn more?
(Other definitions for plain that I've seen before include "Simple, ordinary" , "Not elaborate; clear" , "not striking" , "Geographical feature" , "Treeless expanse" .)