Loud hiccup extremely difficult to sustain (6)
I believe the answer is:
uphold
'sustain' is the definition.
(thesaurus)
'loud hiccup extremely difficult' is the wordplay.
'extremely' suggests removing the centre.
'difficult' indicates anagramming the letters.
'hiccup' with its middle removed is 'hp'.
'loud'+'hp'='loudhp'
'loudhp' is an anagram of 'UPHOLD'.
'to' is the link.
(Other definitions for uphold that I've seen before include "Confirm or support, say a verdict" , "Champion" , "Of appeal court, endorse earlier decision" , "'Support, the law say (6)'" , "Support a decision or belief" .)