Rowing team is hiding key for a decade (8)
I believe the answer is:
eighties
'for a decade' is the definition.
(eighties is a kind of decade)
'rowing team is hiding key' is the wordplay.
'rowing team' becomes 'eight' (eight rowers).
'hiding' means one lot of letters goes inside another.
'key' becomes 'e' (a musical key).
'is' placed around 'e' is 'ies'.
'eight'+'ies'='EIGHTIES'
(Other definitions for eighties that I've seen before include "Period" , "Thatcher's PM decade" , "Eighty to eighty nine" , "some years" .)