Second squad has old power (5)
I believe the answer is:
steam
'old power' is the definition.
'second squad' is the wordplay.
'second' becomes 's' (common abbreviation).
'squad' becomes 'team' (synonyms).
's'+'team'='STEAM'
'has' is the link.
(Other definitions for steam that I've seen before include "Gas formed by vaporizing water at 100?C" , "Invisible gas from boiling water" , "Vapour from water at boiling point" , "Travel in liner" , "Old-fashioned power" .)