Think about old barrier involving it (8)
I believe the answer is:
cogitate
'think' is the definition.
(synonyms)
'about old barrier involving it' is the wordplay.
'about old' becomes 'co' (I've seen this in other clues).
'barrier' becomes 'gate' (gate is a kind of barrier).
'involving' indicates putting letters inside.
'gate' enclosing 'it' is 'gitate'.
'co'+'gitate'='COGITATE'
(Other definitions for cogitate that I've seen before include "Think deeply and long" , "Think, meditate" , "Consider" , "Deliberate" , "Think deeply, ponder" .)