Solemn and old, want to keep lucid in the head (6)
I believe the answer is:
owlish
'solemn' is the definition.
(I've seen this before)
'old want to keep lucid in the head' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'want' becomes 'wish' (synonyms).
'to keep' indicates putting letters inside.
'in the head' indicates taking the first letters.
The initial letter of 'lucid' is 'l'.
'wish' placed around 'l' is 'wlish'.
'o'+'wlish'='OWLISH'
'and' is the link.
(Other definitions for owlish that I've seen before include "Having appearance like wise old bird" , "solemn and wise in appearance?" , "Solemn, wise; like a bird" .)