Old face finally wrinkles up (6)
I believe the answer is:
senile
'old' is the definition.
('senile' can be similar in meaning to 'old')
'face finally wrinkles up' is the wordplay.
'finally' says to take the final letters.
'wrinkles' becomes 'lines' ('line' can be a synonym of 'wrinkle').
'up' is a reversal indicator (in down clue: letters go upwards).
The final letter of 'face' is 'e'.
'e'+'lines'='elines'
'elines' written backwards gives 'SENILE'.
(Other definitions for senile that I've seen before include "Mentally infirm with age" , "Suffering from ailments of old age" , "In one's dotage" , "Mentally weak due to old age" , "Old and doddery" .)