When the call is conclusively on hold (6)
I believe the answer is:
nelson
'hold' is the definition.
(wrestling hold)
'when the call is conclusively on' is the wordplay.
'conclusively' suggests the final letters (the letter which concludes the word).
The last letters of 'when the call is' are 'nels'.
'nels'+'on'='NELSON'
(Other definitions for nelson that I've seen before include "Lady Hamilton's seafaring lover" , "one visibly affected by fighting" , "Naval hero who died at Trafalgar" , "One-eyed English admiral seen on pillar" , "Sailor" .)