Notice European harbour feature accommodating 100 - that’s competence (8)
I believe the answer is:
adequacy
'competence' is the definition.
(I have seen 'Acceptable competence' mean 'adequacy' so perhaps 'competence' could also mean 'adequacy')
'notice european harbour feature accommodating 100' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'european' becomes 'e' (abbreviation e.g. EU).
'harbour feature' becomes 'quay'.
'accommodating' means one lot of letters goes inside another.
'100' becomes 'c' (roman numeral).
'quay' placed around 'c' is 'quacy'.
'ad'+'e'+'quacy'='ADEQUACY'
'that's' is the link.
(Other definitions for adequacy that I've seen before include "It's enough" , "Sufficiency" , "Acceptable competence" , "Sufficient amount" .)