Hell uncovered during show that's unseemly (10)
I believe the answer is:
indelicate
'unseemly' is the definition.
(thesaurus)
'hell uncovered during show' is the wordplay.
'uncovered' says to take the centre.
'during' is an insertion indicator.
'show' becomes 'indicate' ('indicate' can be a synonym of 'show'**).
The centre of 'hell' is 'el'.
'el' put within 'indicate' is 'INDELICATE'.
'that's' is the link.
(Other definitions for indelicate that I've seen before include "lacking taste" , "Saucy" , "Rude - tasteless" , "Coarse" , "Rather rude" .)