Uncovered cyan and navy thread (4)
I believe the answer is:
yarn
'thread' is the definition.
(I know that thread can be written as yarn)
'uncovered cyan and navy' is the wordplay.
'uncovered' says to take the centre.
'and' says to put letters next to each other.
'navy' becomes 'RN' (abbreviation for Royal Navy).
The centre of 'cyan' is 'ya'.
'ya'+'rn'='YARN'
(Other definitions for yarn that I've seen before include "Long involved story" , "which could unravel?" , "Sailor's tale" , "Sham" , "Fabricated" .)