Resolute Republican blocking fighter on the way up (4)
I believe the answer is:
grim
'resolute' is the definition.
(I've seen this before)
'republican blocking fighter on the way up' is the wordplay.
'republican' becomes 'r' (abbreviation).
'blocking' indicates putting letters inside.
'fighter' becomes 'mig' (I've seen this in other clues).
'on the way up' shows that the letters should be reversed in order.
'r' placed inside 'mig' is 'mirg'.
'mirg' written backwards gives 'GRIM'.
(Other definitions for grim that I've seen before include "Very serious/gloomy" , "(Of humour) black; unappealing, depressing" , "The - Reaper is death" , "Dark, depressing" , "Harshly ironic" .)