Current mariners preserving old piece of mast (5)
I believe the answer is:
acorn
'piece of mast' is the definition.
(mast can mean nuts including acorns)
'current mariners preserving old' is the wordplay.
'current' becomes 'ac' (alternating current).
'mariners' becomes 'RN' (Royal Navy).
'preserving' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ac'+'rn'='acrn'
'acrn' placed around 'o' is 'ACORN'.
(Other definitions for acorn that I've seen before include "Fruit of the oak tree" , "mast" , "oak's fruit" , "From which a "tall oak" may grow" , "From which a mighty oak may grow?" .)