Speculate, losing millions in employment (3)
I believe the answer is:
use
'employment' is the definition.
(how something is used)
'speculate losing millions' is the wordplay.
'speculate' becomes 'muse' (synonyms).
'losing' suggests deleting specific letters.
'millions' becomes 'm' (abbreviation).
'muse' with 'm' taken out is 'USE'.
'in' is the link.
(Other definitions for use that I've seen before include "Exploit; custom" , "Exploit unfairly" , "Practise" , "Employ - custom" , "consume; exploit" .)