Ultimately try to fill barrel up wherein soldiers may go for a dip? (4,3)
I believe the answer is:
yolk sac
'soldiers may go for a dip?' is the definition.
(I've seen this before)
'ultimately try to fill barrel up' is the wordplay.
'ultimately' suggests the final letters.
'barrel' becomes 'cask' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
The final letters of 'try to fill' are 'yol'.
'cask' reversed gives 'ksac'.
'yol'+'ksac'='YOLK-SAC'
'wherein' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for yolk sac that I've seen before include "original food parcel" , "bit of egg" , "bag of yellow liquid" , "soldiers may go for a dip" , "Something feeding an embryo" .)