Select broken and occasionally complex instrument (9)
I believe the answer is:
telescope
'instrument' is the definition.
(telescope is a kind of instrument)
'select broken and occasionally complex' is the wordplay.
'broken' is an anagram indicator.
'and' says to put letters next to each other.
'occasionally' indicates alternate letters.
The alternate letters of 'complex' are 'ope'.
'select' is an anagram of 'telesc'.
'telesc'+'ope'='TELESCOPE'
(Other definitions for telescope that I've seen before include "Instrument for viewing distant objects" , "Astronomer's instrument" , "Instrument to magnify the image of distant objects" , "Shrink - optical instrument" , "View distant objects through it" .)