Seen performing at the West End, is nervous (5)
I believe the answer is:
tense
'nervous' is the definition.
('tense' can be similar in meaning to 'nervous')
'seen performing at the west end' is the wordplay.
'performing' is an anagram indicator.
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'end' indicates one should take the final letters.
The final letter of 'west' is 't'.
'seen' is an anagram of 'ense'.
'ense' after 't' is 'TENSE'.
'is' acts as a link.
(Other definitions for tense that I've seen before include "Rigid; verb form" , "Could be present" , "Charged" , "contract" , "Maybe past" .)