Climax with unknown character is seedy (5)
I believe the answer is:
peaky
'seedy' is the definition.
(I've seen this before)
'climax with unknown character' is the wordplay.
'climax' becomes 'peak' (I've seen this in another clue).
'with' says to put letters next to each other.
'unknown character' becomes 'y' (algebra).
'peak'+'y'='PEAKY'
'is' acts as a link.
(Other definitions for peaky that I've seen before include "Rather sickly" , "Seedy" , "Slightly unwell" .)