Tension of pole markers when in service (8)
I believe the answer is:
suspense
'tension' is the definition.
(suspense can mean tension or uncertainty)
'pole markers when in service' is the wordplay.
'pole' becomes 's' (abbreviation for South, as in South Pole).
'markers' becomes 'pens' (marker pens).
'when in' is an insertion indicator.
'service' becomes 'use' (I've seen this in other clues).
'pens' going into 'use' is 'uspense'.
's'+'uspense'='SUSPENSE'
'of' acts as a link.
(Other definitions for suspense that I've seen before include "Feeling of excitement or anxiety about what might happen" , "Pre-climactic excitement" , "The 3 of having to wait" , "State of anxious uncertainty while waiting (8)" , "State of nervous or excited tension or uncertainty" .)