Ageing, but with an unusually solid head of hair (6)

I believe the answer is:
oldish
'ageing' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'with an unusually solid head of hair' is the wordplay.
'with' says to put letters next to each other.
'an unusually' is an anagram indicator.
'head of' suggests taking the first letters.
The initial letter of 'hair' is 'h'.
'solid' anagrammed gives 'oldis'.
'oldis'+'h' is 'OLDISH'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for oldish that I've seen before include "Getting on in years, but not too bad" , "Fairly ancient" .)
