Poet having to write in ship on turning turtle (7)
I believe the answer is:
spenser
'poet' is the definition.
(16th-century poet Edmund Spenser)
'to write in ship on turning turtle' is the wordplay.
'to write' becomes 'pen' ('pen' can be a synonym of 'write').
'in' indicates putting letters inside.
'ship' becomes 'ss'.
'on' becomes 're' ('on' can be mean 'with reference to').
'turning turtle' is a reversal indicator (to turn turtle is to turn upside down).
're' written backwards gives 'er'.
'pen' going inside 'ss' is 'spens'.
'spens'+'er'='SPENSER'
'having' is the link.
(Other definitions for spenser that I've seen before include "Edmund -, Eng. poet" , "Edmund, 16th century poet" , "Edmund ---, Eliz. poet" , "Old English poet" , "He wrote" .)