A lot of sailors secure firm part of mast (5)
I believe the answer is:
acorn
'part of mast' is the definition.
(mast is a mixture of nuts fed to pigs)
'a lot of sailors secure firm' is the wordplay.
'lot of sailors' becomes 'RN' (Royal Navy).
'secure' is an insertion indicator (inserted letters are secured inside).
'firm' becomes 'co' (a firm is a company).
'a'+'rn'='arn'
'arn' going around 'co' is 'ACORN'.
(Other definitions for acorn that I've seen before include "Might source" , "Source of a great oak" , "Nut of oak tree" , "Fruit of the oak tree" , "fruit in a cup" .)