Swedish scientist is retired tennis champion (7)
I believe the answer is:
sievert
'swedish scientist' is the definition.
(physicist Rolf Sievert)
'is retired tennis champion' is the wordplay.
'retired' is a reversal indicator.
'tennis champion' becomes 'evert' (I've seen this before).
'is' in reverse letter order is 'si'.
'si'+'evert'='SIEVERT'
(Other definitions for sievert that I've seen before include "Swedish physicist" , "Measure of radiation dose" , "Physics unit" .)