Too lax training large amphibian (7)
I believe the answer is:
axolotl
'amphibian' is the definition.
(I know that axolotl is a type of ambystomid)
'too lax training large' is the wordplay.
'training' indicates an anagram (to train can mean to grow something into a shape).
'large' becomes 'L' (eg in clothes sizes).
'too'+'lax'='toolax'
'toolax' with letters rearranged gives 'axolot'.
'axolot'+'l'='AXOLOTL'
(Other definitions for axolotl that I've seen before include "Type of salamander" , "amphibian" , "Mexican native" , "Aquatic salamander of N America" , "Mexican salamander - allot ox (anag)" .)