Obvious unhappiness initially about introductory piece (8)
I believe the answer is:
overture
'introductory piece' is the definition.
(I've seen this before)
'obvious unhappiness initially about' is the wordplay.
'obvious' becomes 'overt' (similar in meaning).
'initially' says to take the initial letters.
'about' becomes 're' ('re' means regarding or about).
The initial letter of 'unhappiness' is 'u'.
'overt'+'u'+'re'='OVERTURE'
(Other definitions for overture that I've seen before include "Orchestral opener" , "Opera opening" , "Approach made with view to starting relationship" , "Orchestral introduction to an opera" , "used in attempt to seduce?" .)