Old engine-driver, second to join crew (5)
I believe the answer is:
steam
'old engine' is the definition.
'steam' can be an answer for 'engine' (steam engine is a kind of engine). I'm unsure of the 'old' bit.
'driver second to join crew' is the wordplay.
'driver second' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'to join' means one lot of letters go next to another.
'crew' becomes 'team' (both can mean a group of colleagues).
's'+'team'='STEAM'
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Momentum - boiled water" , "Vaporised water" , "Energy (colloq.)" , "comes with a hiss?" , "get very angry" .)