School's first squad showed old power (5)
I believe the answer is:
steam
'old power' is the definition.
'school's first squad' is the wordplay.
'school's first' becomes 's' (1st letter of 'school').
'squad' becomes 'team' (synonyms).
's'+'team'='STEAM'
'showed' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Old-fashioned power" , "Type of power" , "Water at boiling temperature diffused in the atmosphere" , "Watery vapour" , "comes with a hiss?" .)