Power of second crew (5)
I believe the answer is:
Here is my best explanation:
'power' is the definition.
'second crew' is the subsidiary indication.
'second' becomes 's' (common abbreviation).
'crew' becomes 'team' (I've seen this in other clues).
'of' is the link.
(Other definitions for steam that I've seen before include "Very hot water?", "Source of power", "Vapour from water at boiling temperature", "Source of energy", "Vapour; energy", "Gas formed by vaporizing water at 100?C", "Vaporous exhalation", "Vaporised water", "Water-vapour" and "Hot water vapour".)