Mole crossing river should get lively (4)
I believe the answer is:
spry
'lively' is the definition.
(I've seen this before)
'mole crossing river' is the wordplay.
'mole' becomes 'spy' (I've seen this before).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation for river).
'spy' placed around 'r' is 'SPRY'.
'should get' acts as a link.
(Other definitions for spry that I've seen before include "Alert" , "Far from doddering" , "Nimble, brisk" , "Sprightly" , "Still able to move lightly and fast" .)