He thinks of what he or I served (9)
I believe the answer is:
theoriser
'he thinks' is the definition.
(I've seen this before)
'of what he or i served' is the wordplay.
'of' indicates the answer is hidden within the clue.
'THEORISER' is hidden within 'what he or i served'.
(Other definitions for theoriser that I've seen before include "One who has convictions" , "one who speculates" , "speculator" , "One who reasons" , "he thinks" .)