Roughly calculate what’s left: about one metre (8)
I believe the answer is:
estimate
'roughly calculate' is the definition.
(I've seen this before)
'what's left about one metre' is the wordplay.
'what's left' becomes 'estate' (what's left behind by a deceased person).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'metre' becomes 'm' (synonyms).
'i'+'m'='im'
'estate' going around 'im' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Judgement of approximate value or size" , "Reckon, judge" , "Statement giving the likely cost of a job" , "Evaluate - approximation" , "Judge the approximate value" .)