The writer’s heading for Pamplona, taking on the Spanish force (5)
I believe the answer is:
impel
'force' is the definition.
(synonyms)
'the writer's heading for pamplona taking on the spanish' is the wordplay.
'the writer's' becomes 'I'm' (both could mean 'the crossword setter is').
'heading for' indicates taking the first letters.
'taking on' says to put letters next to each other.
'the spanish' becomes 'el' ('the' in Spanish).
The initial letter of 'pamplona' is 'p'.
'im'+'p'+'el'='IMPEL'
(Other definitions for impel that I've seen before include "Send or drive forward" , "Cause to move forward" , "Urge or force to action" , "Egg" , "'Drive, force (5)'" .)