Poet: one’s range covering start of sonnet (7)
I believe the answer is:
iambist
'poet' is the definition.
(I've seen this before)
'one's range covering start of sonnet' is the wordplay.
'one' becomes 'i' (Roman numeral).
'range' becomes 'ambit' (synonyms).
'covering' indicates putting letters inside.
'start of' says to take the initial letters.
The first letter of 'sonnet' is 's'.
'ambit' placed around 's' is 'ambist'.
'i'+'ambist'='IAMBIST'
(Other definitions for iambist that I've seen before include "poet" , "Writer of verse" , "foot specialist" .)