Outdated second squad (5)
I believe the answer is:
steam
'outdated second squad' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'second squad' is the wordplay.
'second' becomes 's' (common abbreviation).
'squad' becomes 'team' ('team' can be a synonym of 'squad').
's'+'team'='STEAM'
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Form of water" , "Gas formed by vaporizing water at 100?C" , "enthusiasm" , "Travel in liner" , "Pre-digital" .)