Guessing discharge has uranium? Get on the phone! (9)
I believe the answer is:
venturing
'guessing' is the definition.
('venture' can be a synonym of 'guess')
'discharge has uranium? get on the phone' is the wordplay.
'discharge' becomes 'vent' (both can mean to expel or give out).
'has' means one lot of letters go next to another.
'uranium?' becomes 'u' (U is the chemical symbol for uranium).
'get on the phone' becomes 'ring' (to ring someone is to get them on the phone).
'vent'+'u'+'ring'='VENTURING'
(Other definitions for venturing that I've seen before include "Daring" , "Taking a risk" .)