Sequence to work out (5)
I believe the answer is:
train
I believe this is a double definition.
'sequence' is the first definition.
(I've seen this before)
'work out' is the second definition.
(training is a kind of working out)
'to' acts as a link.
(Other definitions for train that I've seen before include "Locomotive plus carriages" , "Teach; series of gears" , "Help (someone) to become fit" , "held up in union action?" , "become physically fit" .)