Occasionally gearbox can be mounted again (6)
I believe the answer is:
encore
'again' is the definition.
(I've seen this before)
'occasionally gearbox can be mounted' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
'gearbox'+'can'+'be'='gearboxcanbe'
The alternating letters of 'gearboxcanbe' are 'erocne'.
'erocne' reversed gives 'ENCORE'.
(Other definitions for encore that I've seen before include "Call for repetition" , "Let's hear/see it again!" , "Repetition of performance" , "Extra item at end of concert" , "Call for additional performance" .)