Underline M representing magnitude in report (9)
I believe the answer is:
emphasise
'underline m' is the definition.
'emphasise' can be an answer for 'underline' ('underline'->'emphasize' is in my internal thesaurus and 'ize'->'ise'). I'm unsure of the 'm' bit.
'magnitude in report' is the wordplay.
'magnitude' becomes 'emphasis' (emphasis is a kind of magnitude).
'in report' becomes 'e' (I can't explain this - if you can you should believe this answer much more).
'emphasis'+'e'='EMPHASISE'
'representing' is the link.
Can you help me to learn more?
(Other definitions for emphasise that I've seen before include "'Stress, accentuate (9)'" , "Mashes pie (anag.)" , "Place stress on" , "Stress, underline" , "Give extra weight to, stress" .)