Expert support -- seamer keeps up (7)
I believe the answer is:
maestro
'expert' is the definition.
(I've seen this before)
'support seamer keeps up' is the wordplay.
'keeps' indicates the answer is hidden within the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'ortseam' can be found hidden inside 'support seamer'.
'ortseam' in reverse letter order is 'MAESTRO'.
(Other definitions for maestro that I've seen before include "Expert musician" , "conductor, maybe" , "Master of an art, or a great conductor of music" , "Good conductor" , "Highly skilled musician" .)