Threatening old man initially, I sense (7)
I believe the answer is:
ominous
'threatening' is the definition.
('ominous' can be a synonym of 'threatening')
'old man initially i sense' is the wordplay.
'initially' indicates taking the first letters.
'sense' becomes 'nous' (nous is a kind of sense**).
The initial letters of 'old man' is 'om'.
'om'+'i'+'nous'='OMINOUS'
(Other definitions for ominous that I've seen before include "Threatening, foreshadowing something bad" , "Looking like trouble coming" , "Minatory, trouble coming" , "'Sinister, baleful (7)'" , "Threatening, doesn't look good" .)