Formal letter, nineteenth one penned by physician today ultimately (6)
I believe the answer is:
dressy
'formal' is the definition.
(I've seen this in another clue)
'letter nineteenth one penned by physician today ultimately' is the wordplay.
'letter nineteenth one' becomes 'ess' (19th letter of the alphabet).
'penned by' indicates putting letters inside (inserted letters are penned in).
'physician' becomes 'Dr' (synonyms).
'ultimately' suggests the final letters.
The final letter of 'today' is 'y'.
'dr'+'y'='dry'
'ess' inserted into 'dry' is 'DRESSY'.
(Other definitions for dressy that I've seen before include "Stylish with regard to clothes" , "Formal or elegant" , "Nattily attired, wearing stylish clothes" , "Wearing stylish clothes, elegant (6)" , "swish" .)