Ways to get the ref's back up are growing, naturally (5)
I believe the answer is:
'ways' is the definition.
Although both the answer and definition are plural nouns, I cannot see how one could define the other.
'the ref's back up are growing naturally' is the wordplay.
'back' is a reversal indicator.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'are growing naturally' becomes 'sn' (I can't explain this - if you can you should give a lot more credence to this answer).
'ref' backwards is 'fer'.
'sn' written backwards gives 'ns'.
'to get' is the link.
Can you help me to learn more?
(Other definitions for ferns that I've seen before include "Flowerless plants with feathery or leafy fronds" , "Frondy plants" , "Flowerless plants with feathery green leaves" , "Ancient plants" , "Plant, maidenhair or tree say" .)