Entering southern strait, Norman occasionally is sleepy (9)
I believe the answer is:
somnolent
'sleepy' is the definition.
(I've seen this before)
'entering southern strait norman occasionally' is the wordplay.
'entering' is an insertion indicator.
'southern strait' becomes 'Solent' (The Solent is a strait in the south of England).
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternating letters of 'norman' are 'omn'.
'solent' enclosing 'omn' is 'SOMNOLENT'.
'is' is the link.
(Other definitions for somnolent that I've seen before include "Drowsy - lost on men (anag)" , "Comatose" , "drifting off" , "Dozy" , "Lent moons (anag.)" .)