To surpass in work, I trust mobile (8)
I believe the answer is:
outstrip
'to surpass' is the definition.
('outstrip' can be a synonym of 'surpass')
'in work i trust mobile' is the wordplay.
'in' is an insertion indicator.
'work' becomes 'op' (abbreviation for opus**).
'mobile' indicates an anagram.
'i'+'trust'='itrust'
'itrust' is an anagram of 'utstri'.
'op' placed around 'utstri' is 'OUTSTRIP'.
(Other definitions for outstrip that I've seen before include "Exceed, overtake" , "Go faster or do better than" , "Leave behind" , "Move faster than and overtake" , "Exceed; move faster than" .)