Sailor and soldier are at first following oddly ugly poisonous creature (9)
I believe the answer is:
tarantula
'oddly ugly poisonous creature' is the definition.
'tarantula' can be an answer for 'creature' (tarantula is a kind of creature). I'm not sure about the 'oddly ugly poisonous' bit.
'sailor and soldier are at first following' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'and' means one lot of letters go next to another.
'soldier' becomes 'ant' (I've seen this before).
'are at' says to put letters next to each other.
'first following' becomes 'ula' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tar'+'ant'+'ula'='TARANTULA'
Can you help me to learn more?
(Other definitions for tarantula that I've seen before include "Hairy arachnid" , "small, venomous creature" , "Large venomous spider" , "web developer" , "A large hairy tropical spider" .)