Able to be conveyed to Barnstaple somehow, crossing river (13)
I believe the answer is:
transportable
'able to be conveyed' is the definition.
(convey can mean to transport)
'to barnstaple somehow crossing river' is the wordplay.
'somehow' indicates an anagram.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation).
'to'+'barnstaple'='tobarnstaple'
'tobarnstaple' with letters rearranged gives 'tansportable'.
'tansportable' going around 'r' is 'TRANSPORTABLE'.
(Another definition for transportable that I've seen is " Such as may be moved".)