I gave Reg fluid for pain (8)
I believe the answer is:
aggrieve
'pain' is the definition.
(thesaurus)
'i gave reg fluid' is the wordplay.
'fluid' indicates an anagram.
'i'+'gave'+'reg'='igavereg'
'igavereg' anagrammed gives 'AGGRIEVE'.
'for' acts as a link.
(Other definitions for aggrieve that I've seen before include "Distress (someone)" , "Distress, afflict" , "Cause pain to; distress" , "Cause offence or injury to" , "Press heavily upon; distress" .)