Unemotional old boy in trio ruined opening of concert (7)
I believe the answer is:
robotic
'unemotional' is the definition.
(I've seen this before)
'old boy in trio ruined opening of concert' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'in' means one lot of letters goes inside another.
'ruined' indicates anagramming the letters.
'opening of' suggests taking the first letters.
The first letter of 'concert' is 'c'.
'trio' anagrammed gives 'roti'.
'ob' placed inside 'roti' is 'roboti'.
'roboti'+'c'='ROBOTIC'
(Other definitions for robotic that I've seen before include "Like a mechanical man" , "Like a machine man" , "Like a mechanical figure" , "Mechanical; unemotional" , "Using automatic" .)