Less worried about old flatfish (5)
I believe the answer is:
soles
'flatfish' is the definition.
Although both the answer and definition are nouns, they are not in the same form of the noun.
'less worried about old' is the wordplay.
'worried' is an anagram indicator.
'about' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'less' anagrammed gives 'sles'.
'sles' going around 'o' is 'SOLES'.
Can you help me to learn more?
(Other definitions for soles that I've seen before include "Parts of shoes" , "Undersides of feet" , "Undersurfaces of feet" , "They're always under one's feet!" , "Fish; bottoms of feet" .)