Technical and prescient, if iconic, externally (10)
I believe the answer is:
scientific
'technical' is the definition.
(thesaurus)
'prescient if iconic externally' is the wordplay.
'prescient' becomes 'scient' (I can't explain this - if you can you should believe this answer much more).
'externally' suggests removing the centre (only the external letters are used).
'iconic' with its middle removed is 'ic'.
'scient'+'if'+'ic'='SCIENTIFIC'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for scientific that I've seen before include "Trained in biology, perhaps" , "Based on observation" , "using what's proven" , "Systematic; methodical" , "As experiment" .)