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' means one lot of letters goes inside another.
'start of' indicates taking the first letters.
The initial letter of 'sonnet' is 's'.
'ambit' going around 's' is 'ambist'.
'i'+'ambist'='IAMBIST'
(Other definitions for iambist that I've seen before include "poet" , "Writer of verse" , "foot specialist" .)