Extremely fickle sailors in plant (4)
I believe the answer is:
fern
'plant' is the definition.
(I know that fern is a type of nonflowering plant)
'extremely fickle sailors' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'fickle' with its centre taken out is 'fe'.
'fe'+'rn'='FERN'
'in' is the link.
(Other definitions for fern that I've seen before include "Flowerless plant with feathery green leaves, common on mountains" , "Plant with featherlike leaves" , "Leafy plant" , "Fronded plant" , "Feathery frondy plant" .)