Listen hard to old ship (4)
I believe the answer is:
hark
'listen' is the definition.
(harking is a kind of listening)
'hard to old ship' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to old ship' becomes 'ark' (as in Noah's ark).
'h'+'ark'='HARK'
(Other definitions for hark that I've seen before include "Hear" , "Listen as in olden days" , "Listen (exclamation)" .)