Strengthens international force entering area of North Sea (9)

'strengthens' is the definition.
('fortify' can be a synonym of 'strengthen')

'international force entering area of north sea' is the wordplay.
'international' becomes 'i' (abbreviation eg in IMF).
'force' becomes 'f' (scientific abbreviation).
'entering' means one lot of letters goes inside another.
'area of north sea' becomes 'forties' (the Long Forties is an area of the North Sea).
'i'+'f'='if'
'if' going within 'forties' is 'FORTIFIES'.