Order helpers in to fill up (9)
I believe the answer is:
replenish
'fill up' is the definition.
(replenishing is a kind of filling up)
'order helpers in' is the wordplay.
'order' is an anagram indicator.
'helpers'+'in'='helpersin'
'helpersin' anagrammed gives 'REPLENISH'.
'to' is the link.
(Other definitions for replenish that I've seen before include "Help siren (anag) - make up" , "Help siren (anag) - fill up again" , "Fill again, renew a supply" , "Stock up again" , "Fill up again, say glass with drink" .)