Locomotive transported by doubly small tugs (7)
I believe the answer is:
strains
'tugs' is the definition.
(straining is a kind of tugging)
'locomotive transported by doubly small' is the wordplay.
I cannot quite see how this works, but
'locomotive' could be 'train' (I've seen this before) and 'train' is found within the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for strains that I've seen before include "Tries very hard" , "Stresses and - -" , "Melodies" .)