Captain Ahab initially runs in from vessel, gaping (9)
I believe the answer is:
cavernous
'gaping' is the definition.
(I've seen this before)
'captain ahab initially runs in from vessel' is the wordplay.
'initially' says to take the initial letters.
'runs' becomes 'r' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'from vessel' becomes 'venous' (from a vein - blood vessel).
The first letters of 'captain ahab' is 'ca'.
'r' inserted within 'venous' is 'vernous'.
'ca'+'vernous'='CAVERNOUS'
(Other definitions for cavernous that I've seen before include "of large underground chambers" , "like a big pit" , "Roomy" , "Gaping" .)