Think back about part (8)
I believe the answer is:
remember
'think back' is the definition.
('remember' can be a synonym of 'think back')
'about part' is the wordplay.
'about' becomes 're' (regarding).
'part' becomes 'member' (member is a kind of part**).
're'+'member'='REMEMBER'
(Other definitions for remember that I've seen before include "Keep in mind" , "Retain memory of" , "Recall from memory" , "Summon back to mind" , "Mark" .)