In the run-up, people pester one (7)
I believe the answer is:
torment
'pester one' is the definition.
'torment' can be an answer for 'pester' (I've seen this before). I'm not certain of the 'one' bit.
'in the run-up people' is the wordplay.
'in' is an insertion indicator.
'the run' becomes 'trot' (trot is a kind of run).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'people' becomes 'men' (men are people).
'trot' written backwards gives 'tort'.
'tort' going around 'men' is 'TORMENT'.
Can you help me to learn more?
(Other definitions for torment that I've seen before include "Pester annoyingly" , "Punish" , "Severe pain" , "Physical or mental pain" , "Anguish" .)