They show bones of unknown number of fish (1-4)
I believe the answer is:
x-rays
'they show bones of unknown' is the definition.
The definition suggests an adverb but the answer is not.
'number of fish' is the wordplay.
I cannot quite understand how this works, but
'number' could be 'x' (X is an example) and 'x' is found within the answer.
'fish' could be 'rays' (I've seen this before) and 'rays' is found in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for x-rays that I've seen before include "Short-wavelength radiation" , "Electromagnetic waves, for medical diagnoses, say" , "Medical images" , "Medical photographs" , "Photos or exams made with kind of radiation" .)