One ordinary seaman penning thank you letters (5)
I believe the answer is:
iotas
'letters' is the definition.
(iota is a kind of letter)
'one ordinary seaman penning thank you' is the wordplay.
'one' becomes 'i' (Roman numeral).
'ordinary seaman' becomes 'os'.
'penning' means one lot of letters goes inside another.
'thank you' becomes 'ta' (informal way of saying thank you).
'os' enclosing 'ta' is 'otas'.
'i'+'otas'='IOTAS'
(Other definitions for iotas that I've seen before include "Jots, very small amounts" , "Tiny scraps" , "Tiny amounts" .)