Obscure muscles displayed by acrobat's last trick (8)
I believe the answer is:
abstruse
'obscure' is the definition.
('abstruse' can be a synonym of 'obscure')
'muscles displayed by acrobat's last trick' is the wordplay.
'muscles' becomes 'abs' (I've seen this before**).
'displayed by' means one lot of letters go next to another.
'last' suggests the final letters.
'trick' becomes 'ruse' (I've seen this before).
The last letter of 'acrobat' is 't'.
'abs'+'t'+'ruse'='ABSTRUSE'
(Other definitions for abstruse that I've seen before include "Incomprehensible to those with normal understanding" , "Hard to understand, perhaps profound" , "Difficult to comprehend" , "Strange busts are difficult to understand" , "Obscure or recondite" .)